qed/qed_ll2: Replace one-element array with flexible-array member

qed/qed_ll2: 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 a flexible-array member in
struct qed_ll2_tx_packet, instead of a one-element array and use the
struct_size() helper to calculate the size for the allocations. Commit
f5823fe6897c ("qed: Add ll2 option to limit the number of bds per packet")
was used as a reference point for these changes.

Also, it's important to notice that flexible-array members should occur
last in any structure, and structures containing such arrays and that
are members of other structures, must also occur last in the containing
structure. That's why _cur_completing_packet_ is now moved to the bottom
in struct qed_ll2_tx_queue. _descq_mem_ and _cur_send_packet_ are also
moved for unification.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays

Tested-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/5f707198.PA1UCZ8MYozYZYAR%25lkp@intel.com/
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>