[PATCH] fix kmem_cache_size() for new slab poisoning

The new slab poisoning code broke kmem_cache_size(), it now returns
a too large size as the poisoning area after the object is includes.
XFS's kmem_zone_zalloc thus overwrites exactly that area and triggers
the new checks everytime such an object is freed again.

I don't recommend using XFS on BK-current without this patch applied :)

mm/slab.c

@@ -2041,11 +2041,16 @@ kfree_percpu(const void *objp)
 
 unsigned int kmem_cache_size(kmem_cache_t *cachep)
 {
+	unsigned int objlen = cachep->objsize;
+
 #if DEBUG
 	if (cachep->flags & SLAB_RED_ZONE)
-		return (cachep->objsize - 2*BYTES_PER_WORD);
+		objlen -= 2*BYTES_PER_WORD;
+	if (cachep->flags & SLAB_STORE_USER)
+		objlen -= BYTES_PER_WORD;
 #endif
-	return cachep->objsize;
+
+	return objlen;
 }
 
 kmem_cache_t * kmem_find_general_cachep (size_t size, int gfpflags)