s390: expose number of page table levels

s390: expose number of page table levels

We would want to use number of page table level to define mm_struct.
Let's expose it as CONFIG_PGTABLE_LEVELS.

Core mm expects __PAGETABLE_{PUD,PMD}_FOLDED to be defined if these page
table levels folded.

Signed-off-by: Kirill A. Shutemov <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Heiko Carstens <[email protected]>
Tested-by: Guenter Roeck <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>