These counters show the number of sockets used for each protocol,
but currently they gather global statistics. This is useful to
have them work per-net.

This set adds this possibility, but hides it under CONFIG_NET_NS
option, so that not-virtualized kernel keeps using the optimized
version by Eric (and he's in Cc for that reason).

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>