MIB: add struct net to UDP6_INC_STATS_USER

As simple as the patch #1 in this set.

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