s390/mm: fix phys vs virt confusion in pgtable allocation routines

The physical address of page tables is passed around and
used as virtual address in various locations.

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