netfilter: conntrack: adjust stop timestamp to real expiry value

In case the entry is evicted via garbage collection there is
delay between the timeout value and the eviction event.

This adjusts the stop value based on how much time has passed.

Fixes: b87a2f91 ("netfilter: conntrack: add gc worker to remove timed-out entries")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>