powerpc/mm: Fix typo in PTRS_PER_PUD

PTRS_PER_PUD should be based on PUD_INDEX_SIZE, not PMD_INDEX_SIZE.  We
got away with it because PUD and PMD had the same index size, but this is
no longer true with Aneesh's patchset to support a 46-bit user effective
address space.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>