slab: Fix missing DEBUG_SLAB last user

Even with SLAB_RED_ZONE and SLAB_STORE_USER enabled, kernel would NOT store
redzone and last user data around allocated memory space if "arch cache line >
sizeof(unsigned long long)". As a result, last user information is unexpectedly
MISSED while dumping slab corruption log.

This fix makes sure that redzone and last user tags get stored unless the
required alignment breaks redzone's.
Signed-off-by: Shiyong Li <shi-yong.li@motorola.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

mm/slab.c

@@ -2220,8 +2220,8 @@ kmem_cache_create (const char *name, size_t size, size_t align,
 	if (ralign < align) {
 		ralign = align;
 	}
-	/* disable debug if necessary */
-	if (ralign > __alignof__(unsigned long long))
+	/* disable debug if not aligning with REDZONE_ALIGN */
+	if (ralign & (__alignof__(unsigned long long) - 1))
 		flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
 	/*
 	 * 4) Store it.
@@ -2247,8 +2247,8 @@ kmem_cache_create (const char *name, size_t size, size_t align,
 	 */
 	if (flags & SLAB_RED_ZONE) {
 		/* add space for red zone words */
-		cachep->obj_offset += sizeof(unsigned long long);
-		size += 2 * sizeof(unsigned long long);
+		cachep->obj_offset += align;
+		size += align + sizeof(unsigned long long);
 	}
 	if (flags & SLAB_STORE_USER) {
 		/* user store requires one word storage behind the end of