net/mlx5e: Fix build warning, detected write beyond size of field

When merged with Linus tree, the cited patch below will cause the
following build warning:

In function 'fortify_memset_chk',
    inlined from 'mlx5e_xmit_xdp_frame' at drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c:438:3:
include/linux/fortify-string.h:242:25: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
  242 |                         __write_overflow_field(p_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix that by grouping the fields to memeset in struct_group() to avoid
the false alarm.

Fixes: 9ded70f ("net/mlx5e: Don't prefill WQEs in XDP SQ in the multi buffer mode")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Suggested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Link: https://lore.kernel.org/r/20220322172224.31849-1-saeed@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c

@@ -435,8 +435,7 @@ mlx5e_xmit_xdp_frame(struct mlx5e_xdpsq *sq, struct mlx5e_xmit_data *xdptxd,
 		u8 num_pkts = 1 + num_frags;
 		int i;
 
-		memset(&cseg->signature, 0, sizeof(*cseg) -
-		       sizeof(cseg->opmod_idx_opcode) - sizeof(cseg->qpn_ds));
+		memset(&cseg->trailer, 0, sizeof(cseg->trailer));
 		memset(eseg, 0, sizeof(*eseg) - sizeof(eseg->trailer));
 
 		eseg->inline_hdr.sz = cpu_to_be16(inline_hdr_sz);

include/linux/mlx5/qp.h

@@ -202,6 +202,9 @@ struct mlx5_wqe_fmr_seg {
 struct mlx5_wqe_ctrl_seg {
 	__be32			opmod_idx_opcode;
 	__be32			qpn_ds;
+
+	struct_group(trailer,
+
 	u8			signature;
 	u8			rsvd[2];
 	u8			fm_ce_se;
@@ -211,6 +214,8 @@ struct mlx5_wqe_ctrl_seg {
 		__be32		umr_mkey;
 		__be32		tis_tir_num;
 	};
+
+	); /* end of trailer group */
 };
 
 #define MLX5_WQE_CTRL_DS_MASK 0x3f