netns: introduce 'struct net_fill_args'

This is a preparatory work. To avoid having to much arguments for the
function rtnl_net_fill(), a new structure is defined.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>