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

arch/powerpc/include/asm/pgtable-ppc64-4k.h

@@ -19,7 +19,7 @@
 
 #define PTRS_PER_PTE	(1 << PTE_INDEX_SIZE)
 #define PTRS_PER_PMD	(1 << PMD_INDEX_SIZE)
-#define PTRS_PER_PUD	(1 << PMD_INDEX_SIZE)
+#define PTRS_PER_PUD	(1 << PUD_INDEX_SIZE)
 #define PTRS_PER_PGD	(1 << PGD_INDEX_SIZE)
 
 /* PMD_SHIFT determines what a second-level page table entry can map */