netfilter: add and use nf_ct_unconfirmed_destroy

This also removes __nf_ct_unconfirmed_destroy() call from
nf_ct_iterate_cleanup_net, so that function can be used only
when missing conntracks from unconfirmed list isn't a problem.

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