use cancel_delayed_work_sync()

Cc: Ben Dooks <ben-linux@fluff.org>
Cc: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>