s390/mm: Cleanup fault error handling

Combine the two VM_FAULT_ERROR checks in do_exception() and move them
to the exit path, similar to x86. Also remove a random blank line.

Suggested-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>