Reading through the code of rtnl_newlink I am perplexed with what your
intended semantics of IFLA_LINK_NETNSID are supposed to be.

My expectation was something with the same semantics IFLA_NET_NS_PID and
IFLA_NET_NS_FD just a different data representation and something that
could be used in more netlink messages, so you could report the network
namespace where the other end of a tunnel or the the network namespace
of an underlying device is.

Being very slow I would expect that ILFA_LINK_NETNSID would replace
dest_net or possibly net in rtnl_newlink but it does not replace either
of those completely.  Which causes me to think that the implementation
of IFLA_LINK_NETNSID in rtnl_newlink is broken.

I suspect the correct fix for rtnl_newlink is to just use
IFLA_LINK_NETNSID in rtnl_link_get_net and have it be an alternative way
of setting dest_net.  But you may intend some different semantics that
I don't understand.

Eric