[NETNS]: Minor information leak via /proc/net/ptype file.

This file displays the registered packet types, but some of them
(packet sockets creates such) can be bound to a net device and showing
them in a wrong namespace is not correct.

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