[DCCP]: Fix comment about control sockets.

These sockets now have a bit other names and are no longer global.

Shame on me, I haven't provided a good comment for this when
sending DCCP netnsization patches.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>