netfilter: nf_conncount: Split insert and traversal

This patch is originally from Florian Westphal.

When we have a very coarse grouping, e.g. by large subnets, zone id,
etc, it's likely that we do not need to do tree rotation because
we'll find a node where we can attach new entry.  Based on this
observation, we split tree traversal and insertion.

Later on, we can make traversal lockless (tree protected
by RCU), and add extra lock in the individual nodes to protect list
insertion/deletion, thereby allowing parallel insert/delete in different
tree nodes.

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>