af_iucv: remove IUCV-pathes completely

A SEVER is missing in the callback of a receiving SEVERED. This may
inhibit z/VM to remove the corresponding IUCV-path completely.
This patch adds a SEVER in iucv_callback_connrej (together with
additional locking.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>