s390/mm: use pfmf instruction to initialize storage keys

Make use of the pfmf instruction, if available, to initialize storage keys
of whole 1MB or 2GB frames instead of initializing every single page with
the sske instruction.

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