Add another allocator to the common mempool code: a kmalloc/kfree allocator

This will be used by the next patch in the series to replace duplicate
mempool-backed kmalloc allocators in several places in the kernel.  It is also
very likely that there will be more users in the future.

Signed-off-by: Matthew Dobson <colpatch@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>