s390/mm: missing pte for gmap_ipte_notify should trigger a VM_BUG

After fixup_user_fault does not fail we have a writeable pte.
That pte might transform but it should not vanish.
Signed-off-by: NDominik Dingel <dingel@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/pgtable.c

@@ -752,8 +752,7 @@ int gmap_ipte_notify(struct gmap *gmap, unsigned long gaddr, unsigned long len)
 			break;
 		/* Walk the process page table, lock and get pte pointer */
 		ptep = get_locked_pte(gmap->mm, addr, &ptl);
-		if (unlikely(!ptep))
-			continue;
+		VM_BUG_ON(!ptep);
 		/* Set notification bit in the pgste of the pte */
 		entry = *ptep;
 		if ((pte_val(entry) & (_PAGE_INVALID | _PAGE_PROTECT)) == 0) {