netfilter: conntrack: destroy functions need to free queued packets

queued skbs might be using conntrack extensions that are being removed,
such as timeout.  This happens for skbs that have a skb->nfct in
unconfirmed state (i.e., not in hash table yet).

This is destructive, but there are only two use cases:
 - module removal (rare)
 - netns cleanup (most likely no conntracks exist, and if they do,
   they are removed anyway later on).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>