s390/mm: Allocate page table with PAGE_SIZE granularity

Make vmem_pte_alloc() consistent by always allocating page table of
PAGE_SIZE granularity, regardless of whether page_table_alloc() (with
slab) or memblock_alloc() is used. This ensures page table can be fully
freed when the corresponding page table entries are removed.

Fixes: d08d4e7c ("s390/mm: use full 4KB page for 2KB PTE")
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>