netfilter: ipt_CLUSTERIP: make proc directory per net namespace

Create /proc/net/ipt_CLUSTERIP directory for per net namespace.
Right now,only allow to create entries under the ipt_CLUSTERIP
in init net namespace.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>