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

net/netfilter/xt_IDLETIMER.c

@@ -236,6 +236,7 @@ static void idletimer_tg_destroy(const struct xt_tgdtor_param *par)
 
 		list_del(&info->timer->entry);
 		del_timer_sync(&info->timer->timer);
+		cancel_work_sync(&info->timer->work);
 		sysfs_remove_file(idletimer_tg_kobj, &info->timer->attr.attr);
 		kfree(info->timer->attr.attr.name);
 		kfree(info->timer);