s390/mm,fault: replace WARN_ON_ONCE() with unreachable()

do_secure_storage_access() contains a switch statements which handles
all possible return values from get_fault_type(). Therefore remove the
pointless default case error handling and replace it with unreachable().

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>