xprtrdma: Fix helper that drains the transport

We want to drain only the RQ first. Otherwise the transport can
deadlock on ->close if there are outstanding Send completions.

Fixes: 6d2d0ee ("xprtrdma: Replace rpcrdma_receive_wq ... ")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: stable@vger.kernel.org # v5.0+
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/xprtrdma/verbs.c

@@ -90,7 +90,7 @@ static void rpcrdma_xprt_drain(struct rpcrdma_xprt *r_xprt)
 	/* Flush Receives, then wait for deferred Reply work
 	 * to complete.
 	 */
-	ib_drain_qp(ia->ri_id->qp);
+	ib_drain_rq(ia->ri_id->qp);
 	drain_workqueue(buf->rb_completion_wq);
 
 	/* Deferred Reply processing might have scheduled