af_iucv: add shutdown for HS transport

AF_IUCV sockets offer a shutdown function. This patch makes sure
shutdown works for HS transport as well.

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