At present our slab debugging tells us that it detected a double-free or
corruption - it does not distinguish between them.  Sometimes it's useful
to be able to differentiate between these two types of information.

Add double-free detection to redzone verification when freeing an object.
As explained by Manfred, when we are freeing an object, both redzones
should be RED_ACTIVE.  However, if both are RED_INACTIVE, we are trying to
free an object that was already free'd.
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>