[NETNS]: Add an empty netns_dccp structure on struct net.

According to the overall struct net design, it will be
filled with DCCP-related members.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>