powerpc/mm: enable the use of page table cache of order 0

hugepages uses a cache of order 0. Lets allow page tables
of order 0 in the common part in order to avoid open coding
in hugetlb

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/nohash/32/pgalloc.h

@@ -25,10 +25,7 @@
 extern void __bad_pte(pmd_t *pmd);
 
 extern struct kmem_cache *pgtable_cache[];
-#define PGT_CACHE(shift) ({				\
-			BUG_ON(!(shift));		\
-			pgtable_cache[(shift) - 1];	\
-		})
+#define PGT_CACHE(shift) pgtable_cache[shift]
 
 static inline pgd_t *pgd_alloc(struct mm_struct *mm)
 {

arch/powerpc/include/asm/nohash/64/pgalloc.h

@@ -36,10 +36,7 @@ extern struct vmemmap_backing *vmemmap_list;
 #define MAX_PGTABLE_INDEX_SIZE	0xf
 
 extern struct kmem_cache *pgtable_cache[];
-#define PGT_CACHE(shift) ({				\
-			BUG_ON(!(shift));		\
-			pgtable_cache[(shift) - 1];	\
-		})
+#define PGT_CACHE(shift) pgtable_cache[shift]
 
 static inline pgd_t *pgd_alloc(struct mm_struct *mm)
 {