s390/zcore: release dump save area on restart or power down

The zFCP/NVMe standalone dumper is supposed to release the dump save area
resource as soon as possible but might fail to do so, for instance, if it
crashes. To avoid this situation, register a reboot notifier and ensure
the dump save area resource is released on reboot or power down.

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>