svcrdma: Reserve an extra WQE for ib_drain_rq()

Do as other ULPs already do: ensure there is an extra Receive WQE
reserved for the tear-down drain WR. I haven't heard reports of
problems but it can't hurt.

Note that rq_depth is used to compute the Send Queue depth as well,
so this fix should affect both the SQ and RQ.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/xprtrdma/svc_rdma_transport.c

@@ -415,7 +415,7 @@ static struct svc_xprt *svc_rdma_accept(struct svc_xprt *xprt)
 	if (newxprt->sc_max_send_sges > dev->attrs.max_send_sge)
 		newxprt->sc_max_send_sges = dev->attrs.max_send_sge;
 	rq_depth = newxprt->sc_max_requests + newxprt->sc_max_bc_requests +
-		   newxprt->sc_recv_batch;
+		   newxprt->sc_recv_batch + 1 /* drain */;
 	if (rq_depth > dev->attrs.max_qp_wr) {
 		rq_depth = dev->attrs.max_qp_wr;
 		newxprt->sc_recv_batch = 1;