s390/smp: Handle restart interrupt on ipl cpu

s390/smp: Handle restart interrupt on ipl cpu

The current smp code allows to trigger a restart interrupt on CPUs
offline in linux. To allow using the percpu infrastructure instead
of the pcpu_devices array, switch to the ipl cpu which is always
online before calling do_restart().

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>