netfilter: flowtables: use fixed renew timeout on teardown

This is one of the very few external callers of ->get_timeouts(),

We can use a fixed timeout instead, conntrack core will refresh this in
case a new packet comes within this period.

Use of ESTABLISHED timeout seems way too huge anyway.

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