powerpc/mm: Fix encoding of page table cache numbers

The mask used to encode the page table cache number in the
batch when freeing page tables was too small for the new
possible values of MMU page sizes. This increases it along
with a comment explaining the constraints.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/pgalloc.h

@@ -28,7 +28,12 @@ typedef struct pgtable_free {
 	unsigned long val;
 } pgtable_free_t;
 
-#define PGF_CACHENUM_MASK	0x7
+/* This needs to be big enough to allow for MMU_PAGE_COUNT + 2 to be stored
+ * and small enough to fit in the low bits of any naturally aligned page
+ * table cache entry. Arbitrarily set to 0x1f, that should give us some
+ * room to grow
+ */
+#define PGF_CACHENUM_MASK	0x1f
 
 static inline pgtable_free_t pgtable_free_cache(void *p, int cachenum,
 						unsigned long mask)