xprtrdma: Optimize rpcrdma_post_recvs()

Micro-optimization: In rpcrdma_post_recvs, since commit e340c2d6
("xprtrdma: Reduce the doorbell rate (Receive)"), the common case is
to return without doing anything. Found with perf.
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>
Signed-off-by: NAnna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/verbs.c

@@ -1465,7 +1465,7 @@ rpcrdma_post_recvs(struct rpcrdma_xprt *r_xprt, bool temp)
 	count = 0;
 
 	needed = buf->rb_credits + (buf->rb_bc_srv_max_requests << 1);
-	if (ep->rep_receive_count > needed)
+	if (likely(ep->rep_receive_count > needed))
 		goto out;
 	needed -= ep->rep_receive_count;
 	if (!temp)