[NETNS][DCCPV4]: Add dummy per-net operations.

They will be responsible for ctl socket initialization, but
currently they are void.

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>