net/mlx5e: Replace 0-length array with flexible array

Zero-length arrays are deprecated[1]. Replace struct mlx5e_rx_wqe_cyc's
"data" 0-length array with a flexible array. Detected with GCC 13,
using -fstrict-flex-arrays=3:

drivers/net/ethernet/mellanox/mlx5/core/en_main.c: In function 'mlx5e_alloc_rq':
drivers/net/ethernet/mellanox/mlx5/core/en_main.c:827:42: warning: array subscript f is outside array bounds of 'struct mlx5_wqe_data_seg[0]' [-Warray-bounds=]
  827 |                                 wqe->data[f].byte_count = 0;
      |                                 ~~~~~~~~~^~~
In file included from drivers/net/ethernet/mellanox/mlx5/core/en/tc_ct.h:11,
                 from drivers/net/ethernet/mellanox/mlx5/core/eswitch.h:48,
                 from drivers/net/ethernet/mellanox/mlx5/core/en_main.c:42:
drivers/net/ethernet/mellanox/mlx5/core/en.h:250:39: note: while referencing 'data'
  250 |         struct mlx5_wqe_data_seg      data[0];
      |                                       ^~~~

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

Cc: Saeed Mahameed <saeedm@nvidia.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: netdev@vger.kernel.org
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

drivers/net/ethernet/mellanox/mlx5/core/en.h

@@ -247,7 +247,7 @@ struct mlx5e_rx_wqe_ll {
 };
 
 struct mlx5e_rx_wqe_cyc {
-	struct mlx5_wqe_data_seg      data[0];
+	DECLARE_FLEX_ARRAY(struct mlx5_wqe_data_seg, data);
 };
 
 struct mlx5e_umr_wqe {