[VLAN]: Make the /proc/net/vlan/conf file show per-net info.

It is created in a proper net, so make is show info, related
to this particular net.

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