s390/mm,fault: use static key for store indication

Generate slightly better code by using a static key to implement store
indication. This allows to get rid of a memory access on the hot path.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>