KVM: s390/mm: try a cow on read only pages for key ops

The PFMF instruction handler  blindly wrote the storage key even if
the page was mapped R/O in the host. Lets try a COW before continuing
and bail out in case of errors.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org