powerpc/mm: Don't open code pgtable_t size

The slot information of base page size hash pte is stored in the
pgtable_t w.r.t transparent hugepage. We need to make sure we don't
index beyond pgtable_t size.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/hugepage-hash64.c

@@ -71,7 +71,7 @@ int __hash_page_thp(unsigned long ea, unsigned long access, unsigned long vsid,
 	 */
 	shift = mmu_psize_defs[psize].shift;
 	index = (ea & ~HPAGE_PMD_MASK) >> shift;
-	BUG_ON(index >= 4096);
+	BUG_ON(index >= PTE_FRAG_SIZE);
 
 	vpn = hpt_vpn(ea, vsid, ssize);
 	hpte_slot_array = get_hpte_slot_array(pmdp);