netfilter: conntrack: add nf_ct_iterate_destroy

sledgehammer to be used on module unload (to remove affected conntracks
from all namespaces).

It will also flag all unconfirmed conntracks as dying, i.e. they will
not be committed to main table.

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