s390/mm,fault: use get_fault_address() everywhere

Use the get_fault_address() helper function instead of open-coding it
at many locations.

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>