netfilter: move nf_ct_netns_get out of nf_conncount_init

netfilter: move nf_ct_netns_get out of nf_conncount_init

This patch is to move nf_ct_netns_get() out of nf_conncount_init()
and let the consumers of nf_conncount decide if they want to turn
on netfilter conntrack.

It makes nf_conncount more flexible to be used in other places and
avoids netfilter conntrack turned on when using it in openvswitch
conntrack.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>