iucv: prevent cpu hotplug when walking cpu_online_map.

The code used preempt_disable() to prevent cpu hotplug, however that
doesn't protect for cpus being added. So use get_online_cpus() instead.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>