s390/fault: use _ASCE_ORIGIN instead of PAGE_MASK

When masking an ASCE to get its origin use the corresponding define
instead of the unrelated PAGE_MASK.
This doesn't fix a bug since both masks are identical.

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