[PATCH] fix kmem_cache_size() for new slab poisoning

[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 :)