MIB: add struct net to UDP_INC_STATS_USER

Nothing special - all the places already have a struct sock
at hands, so use the sock_net() net.

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>