netfilter: nf_conncount: Move locking into count_tree()

This patch is originally from Florian Westphal.

This is a preparation patch to allow lockless traversal
of the tree via RCU.

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>