RDMA/cxgb4: Replace 0-length arrays with flexible arrays

RDMA/cxgb4: Replace 0-length arrays with flexible arrays

Zero-length arrays are deprecated[1]. Replace all remaining
0-length arrays with flexible arrays. Detected with GCC 13, using
-fstrict-flex-arrays=3:

In function 'build_rdma_write',
    inlined from 'c4iw_post_send' at ../drivers/infiniband/hw/cxgb4/qp.c:1173:10:
../drivers/infiniband/hw/cxgb4/qp.c:597:38: warning: array subscript 0 is outside array bounds of 'struct fw_ri_immd[0]' [-Warray-bounds=]
  597 |                 wqe->write.u.immd_src[0].r2 = 0;
      |                 ~~~~~~~~~~~~~~~~~~~~~^~~
../drivers/infiniband/hw/cxgb4/t4fw_ri_api.h: In function 'c4iw_post_send':
../drivers/infiniband/hw/cxgb4/t4fw_ri_api.h:567:35: note: while referencing 'immd_src'
  567 |                 struct fw_ri_immd immd_src[0];
      |                                   ^~~~~~~~

Additionally drop the unused C99_NOT_SUPPORTED ifndef lines.

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

Cc: Potnuri Bharat Teja <bharat@chelsio.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230105223225.never.252-kees@kernel.org
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Leon Romanovsky <leon@kernel.org>