s390/ptdump: add missing amode31 markers

Add amode31 markers which makes a ro mapping in the middle of
nowhere in the kernel_page_tables output less magic.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>