amt: cancel delayed_work synchronously in amt_fini()

amt: cancel delayed_work synchronously in amt_fini()

When the amt module is being removed, it calls cancel_delayed_work()
to cancel pending delayed_work. But this function doesn't wait for
canceling delayed_work.
So, workers can be still doing after module delete.

In order to avoid this, cancel_delayed_work_sync() should be used instead.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Fixes: 410f2274f310 ("amt: add multicast(IGMP) report message handler")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Link: https://lore.kernel.org/r/20211116160923.25258-1-ap420073@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>