netfilter: ipt_CLUSTERIP: make clusterip_list per net namespace

clusterip_configs should be per net namespace, so operate
cluster in one net namespace won't affect other net
namespace. right now, only allow to operate the clusterip_configs
of init net namespace.

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