[NETNS][UDP]: Register /proc/net/udp in a namespace.

After the commit a91275eff43a527e1a25d6d034cbcd19ee323e64 ([NETNS][IPV6]
udp - make proc handle the network namespace) it is now possible to make
this file present in newly created namespaces.

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