netfilter: cttimeout: decouple unlink and free on netns destruction
Make it so netns pre_exit unlinks the objects from the pernet list, so
they cannot be found anymore.
netns core issues a synchronize_rcu() before calling the exit hooks so
any the time the exit hooks run unconfirmed nf_conn entries have been
free'd or they have been committed to the hashtable.
The exit hook still tags unconfirmed entries as dying, this can
now be removed in a followup change.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>