Don't allow rmap to touch reserved or out-of-range pages

mm/memory.c

@@ -292,13 +292,11 @@ skip_copy_pmd_range:	address = (address + PGDIR_SIZE) & PGDIR_MASK;
 				 * and not mapped via rmap - duplicate the
 				 * mapping as is.
 				 */
-				if (!pfn_valid(pfn)) {
+				page = pfn_to_page(pfn);
+				if (!pfn_valid(pfn) || PageReserved(page)) {
 					set_pte(dst_pte, pte);
 					goto cont_copy_pte_range_noset;
 				}
-				page = pfn_to_page(pfn);
-				if (PageReserved(page))
-					goto cont_copy_pte_range;
 
 				/*
 				 * If it's a COW mapping, write protect it both