netfilter: conntrack: reduce timeout when receiving out-of-window fin or rst

In case the endpoints and conntrack go out-of-sync, i.e. there is
disagreement wrt. validy of sequence/ack numbers between conntracks
internal state and those of the endpoints, connections can hang for a
long time (until ESTABLISHED timeout).

This adds a check to detect a fin/fin exchange even if those are
invalid.  The timeout is then lowered to UNACKED (default 300s).

Signed-off-by: Florian Westphal <fw@strlen.de>