s390/memory hotplug: use pfmf instruction to initialize storage keys

Move and rename init_storage_keys() to pageattr.c, so it can also be
used from the sclp memory hotplug code in order to initialize
storage keys.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>