mm: add missing __PAGETABLE_{PUD,PMD}_FOLDED defines

mm: add missing __PAGETABLE_{PUD,PMD}_FOLDED defines

Core mm expects __PAGETABLE_{PUD,PMD}_FOLDED to be defined if these page
table levels folded.  Usually, these defines are provided by
<asm-generic/pgtable-nopmd.h> and <asm-generic/pgtable-nopud.h>.

But some architectures fold page table levels in a custom way.  They
need to define these macros themself.  This patch adds missing defines.

The patch fixes mm->nr_pmds underflow and eliminates dead __pmd_alloc()
and __pud_alloc() on architectures without these page table levels.

Signed-off-by: Kirill A. Shutemov <[email protected]>
Cc: Aaro Koskinen <[email protected]>
Cc: David Howells <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: "James E.J. Bottomley" <[email protected]>
Cc: Koichi Yasutake <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>