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>