netfilter: nft_ct: drop pending enqueued packets on removal

netfilter: nft_ct: drop pending enqueued packets on removal

Packets sitting in nfqueue might hold a reference to:

- templates that specify the conntrack zone, because a percpu area is
  used and module removal is possible.
- conntrack timeout policies and helper, where object removal leave
  a stale reference.

Since these objects can just go away, drop enqueued packets to avoid
stale reference to them.

If there is a need for finer grain removal, this logic can be revisited
to make selective packet drop upon dependencies.

Fixes: 7e0b2b57f01d ("netfilter: nft_ct: add ct timeout support")
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>