net: fddi: skfp: smt: Replace one-element array with flexible-array member

net: fddi: skfp: smt: Replace one-element array with flexible-array member

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code according to the use of flexible-array members in
smt_sif_operation structure, instead of one-element arrays. Also, make
use of the struct_size() helper instead of the open-coded version
to calculate the size of the struct-with-flex-array. Additionally, make
use of the typeof operator to properly determine the object type to be
passed to macro smtod().

Also, this helps the ongoing efforts to enable -Warray-bounds by fixing
the following warnings:

  CC [M]  drivers/net/fddi/skfp/smt.o
drivers/net/fddi/skfp/smt.c: In function ‘smt_send_sif_operation’:
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>