netfilter: ctnetlink: remove expired entries first

When dumping conntrack table to userspace via ctnetlink, check if the ct has
already expired before doing any of the 'skip' checks.

This expires dead entries faster.
/proc handler also removes outdated entries first.

Reported-by: Vitaly Zuevsky <vzuevsky@ns1.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>