proc: create /proc/net/netstat file in each net

Now all the shown in it statistics is netnsizated, time to
show it in appropriate net.

The appropriate net init/exit ops already exist - they make
the sockstat file per net - so just extend them.

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