RDMA/rxe: Use sizeof instead of hard code number

Use 'sizeof(union rdma_network_hdr)' instead of hard code GRH length
for GSI and UD.

Link: https://patch.msgid.link/r/20240822065223.1117056-2-pizhenwei@bytedance.comSigned-off-by: zhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/sw/rxe/rxe_resp.c

@@ -351,7 +351,7 @@ static enum resp_states rxe_resp_check_length(struct rxe_qp *qp,
 
 		for (i = 0; i < qp->resp.wqe->dma.num_sge; i++)
 			recv_buffer_len += qp->resp.wqe->dma.sge[i].length;
-		if (payload + 40 > recv_buffer_len) {
+		if (payload + sizeof(union rdma_network_hdr) > recv_buffer_len) {
 			rxe_dbg_qp(qp, "The receive buffer is too small for this UD packet.\n");
 			return RESPST_ERR_LENGTH;
 		}