slub: use __SetPageSlab function to set PG_slab flag

To set page-flag, using SetPageXXXX() and __SetPageXXXX() is more
understandable and maintainable. So change it.
Signed-off-by: NJoonsoo Kim <js1304@gmail.com>
Signed-off-by: NPekka Enberg <penberg@kernel.org>

mm/slub.c

@@ -1369,7 +1369,7 @@ static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node)
 
 	inc_slabs_node(s, page_to_nid(page), page->objects);
 	page->slab = s;
-	page->flags |= 1 << PG_slab;
+	__SetPageSlab(page);
 
 	start = page_address(page);