iucv: fix kernel panic at reboot

The iucv base layer is initialized during the registration of the
first iucv handler.  If no handler is registered and the
iucv_reboot_event() notifier is called, a missing check can cause
a kernel panic in iucv_block_cpu().  To solve this issue, check the
IRQ masks invoke iucv_block_cpu() for enabled CPUs only.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>