af_iucv: consider state IUCV_CLOSING when closing a socket

Make sure a second invocation of iucv_sock_close() guarantees proper
freeing of an iucv path.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>