netfilter: IDLETIMER: fix race condition when destroy the target

Workqueue maybe still in running while we destroy the IDLETIMER target,
thus cause a use after free error, add cancel_work_sync() to avoid such
situation.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>