netlink: fix lookup check
netlink_unicast() is done in init_net context because
a) rtnl socket is bound to init_net,
b) kernel-space socket is successfully looked up by any VE,
c) rtnl is kernel-spase socket.
which is b-r-o-k-e-n, because e.g. just about any manipulation with
netdevices via netlink will be projected onto VE0.
Fix (after per-netns rtnl socket patches)