[NETNS][TCP]: Register /proc/net/tcp in a namespace.

After the commit f40c8174 ([NETNS][IPV4] 
tcp - make proc handle the network namespaces) 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>