s390/mm,fault: call do_fault_error() only from do_exception()

Remove duplicated fault error handling and handle it only once within
do_exception().

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>