netfilter: nf_conncount: Switch to plain list

Original patch is from Florian Westphal.

This patch switches from hlist to plain list to store the list of
connections with the same filtering key in nf_conncount. With the
plain list, we can insert new connections at the tail, so over time
the beginning of list holds long-running connections and those are
expired, while the newly creates ones are at the end.

Later on, we could probably move checked ones to the end of the list,
so the next run has higher chance to reclaim stale entries in the front.

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>