[S390] appldata: prevent cpu hotplug when walking cpu_online_map.

Use get_online_cpus() to prevent cpu hotplug in situations where
for_each_online_cpu() is called.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>