From: Manfred Spraul <manfred@colorfullife.com>

below is the promised patch for better slab debugging, against 2.5.68-mm4:

Changes:

- enable redzoning and last user accounting even for large objects, if
  that doesn't waste too much memory

- document why FORCED_DEBUG doesn't enable redzoning&last user accounting
  for some caches.

- check the validity of the bufctl chains in a slab in __free_blocks.
  This detects double-free error for the caches without redzoning.