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>

net/iucv/iucv.c

@@ -831,8 +831,11 @@ static int iucv_reboot_event(struct notifier_block *this,
 {
 	int i;
 
+	if (cpumask_empty(&iucv_irq_cpumask))
+		return NOTIFY_DONE;
+
 	get_online_cpus();
-	on_each_cpu(iucv_block_cpu, NULL, 1);
+	on_each_cpu_mask(&iucv_irq_cpumask, iucv_block_cpu, NULL, 1);
 	preempt_disable();
 	for (i = 0; i < iucv_max_pathid; i++) {
 		if (iucv_path_table[i])