Remove all uses of kmem_cache_t (the most were left in slab.h).  The
typedef for kmem_cache_t is then only necessary for other kernel
subsystems.  Add a comment to that effect.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>