xprtrdma: Toggle XPRT_CONGESTED in xprtrdma's slot methods

Commit 48be539d ("xprtrdma: Introduce ->alloc_slot call-out for
xprtrdma") added a separate alloc_slot and free_slot to the RPC/RDMA
transport. Later, commit 75891f50 ("SUNRPC: Support for
congestion control when queuing is enabled") modified the generic
alloc/free_slot methods, but neglected the methods in xprtrdma.

Found via code review.

Fixes: 75891f50 ("SUNRPC: Support for congestion control ... ")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/transport.c

@@ -571,6 +571,7 @@ xprt_rdma_alloc_slot(struct rpc_xprt *xprt, struct rpc_task *task)
 	return;
 
 out_sleep:
+	set_bit(XPRT_CONGESTED, &xprt->state);
 	rpc_sleep_on(&xprt->backlog, task, NULL);
 	task->tk_status = -EAGAIN;
 }
@@ -589,7 +590,8 @@ xprt_rdma_free_slot(struct rpc_xprt *xprt, struct rpc_rqst *rqst)
 
 	memset(rqst, 0, sizeof(*rqst));
 	rpcrdma_buffer_put(&r_xprt->rx_buf, rpcr_to_rdmar(rqst));
-	rpc_wake_up_next(&xprt->backlog);
+	if (unlikely(!rpc_wake_up_next(&xprt->backlog)))
+		clear_bit(XPRT_CONGESTED, &xprt->state);
 }
 
 static bool rpcrdma_check_regbuf(struct rpcrdma_xprt *r_xprt,