It is usefull for daemons that monitor link event to have the full parameters of
these interfaces when a rtnl message is sent.
It allows also to dump them via rtnetlink.

It is based on what is done for GRE tunnels.
Signed-off-by: NNicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>