s390/mm: do not initialize storage keys

With dirty and referenced bits implemented in software it is unnecessary
to initialize the storage key for every page. With this patch not a single
storage key operation is done for a system that does not use KVM.
For KVM set_pte_at/pgste_set_key will do the initialization for the guest
view of the storage key when the mapping for the page is established in
the host.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>