IB/mlx4: Avoid field-overflowing memcpy()

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring array fields.

Use the ether_addr_copy() helper instead, as already done for smac.

Link: https://lore.kernel.org/r/20210616203744.1248551-1-keescook@chromium.orgSigned-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/mlx4/qp.c

@@ -3144,7 +3144,7 @@ static int build_mlx_header(struct mlx4_ib_qp *qp, const struct ib_ud_wr *wr,
 		mlx->sched_prio = cpu_to_be16(pcp);
 
 		ether_addr_copy(sqp->ud_header.eth.smac_h, ah->av.eth.s_mac);
-		memcpy(sqp->ud_header.eth.dmac_h, ah->av.eth.mac, 6);
+		ether_addr_copy(sqp->ud_header.eth.dmac_h, ah->av.eth.mac);
 		memcpy(&ctrl->srcrb_flags16[0], ah->av.eth.mac, 2);
 		memcpy(&ctrl->imm, ah->av.eth.mac + 2, 4);