[NETNS][UDP-Lite]: Register /proc/net/udplite(6) in a namespace.

UDP-Lite sockets are displayed in another files, rather than
UDP ones, so make the present in namespaces as well.

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