af_iucv: avoid path quiesce of severed path in shutdown()

An af_iucv stress test showed -EPIPE results for sendmsg()
calls. They are caused by quiescing a path even though it has
been already severed by peer. For IUCV transport shutdown()
consists of 2 steps:
(1) sending the shutdown message to peer
(2) quiescing the iucv path
If the iucv path between these 2 steps is severed due to peer
closing the path, the quiesce step is no longer needed.
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <blaschka@linux.vnet.ibm.com>
Reported-by: Philipp Hachtmann <phacht@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/iucv/af_iucv.c

@@ -1542,7 +1542,8 @@ static int iucv_sock_shutdown(struct socket *sock, int how)
 
 	sk->sk_shutdown |= how;
 	if (how == RCV_SHUTDOWN || how == SHUTDOWN_MASK) {
-		if (iucv->transport == AF_IUCV_TRANS_IUCV) {
+		if ((iucv->transport == AF_IUCV_TRANS_IUCV) &&
+		    iucv->path) {
 			err = pr_iucv->path_quiesce(iucv->path, NULL);
 			if (err)
 				err = -ENOTCONN;