This argument is not used anymore.

Fixes: cab3c8ec ("netns: always provide the id to rtnl_net_fill()")
Signed-off-by: NNicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>