Create/destroy kcopyd on demand.

This changes kcopyd to initialize its mempool and workqueue only when a
client specifically needs to use it.
Signed-off-by: Kevin Corry <kevcorry@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>