s390/mm: use compound page order to distinguish page tables

CRSTs always have size of four pages, while 2KB-size page tables
always occupy a single page. Use that information to distinguish
page tables from CRSTs.

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