netfilter: cttimeout: decouple unlink and free on netns destruction

Increment the extid on module removal; this makes sure that even
in extreme cases any old uncofirmed entry that happened to be kept
e.g. on nfnetlink_queue list will not trip over a stale timeout
reference.

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