[PATCH] Babel: add RFC9229 (v4 via v6) support

[Juliusz Chroboczek]

> The RFC says "ordinary IPv4 announcements are preferred to v4-via-v6
> announcements when the outgoing interface has an assigned IPv4 address",
> but there is also the third option - using an IPv4 address from another
> interface as a next hop.

The ways in which routing may fail are different.

If you announce a v4-via-v6 route and the peer doesn't support v4-via-v6,
then the v4-via-v6 announcements will be silently ignored, and the routed
packets will either take a longer v4 route, or follow the default route.

If, on the other hand, you announce an IPv4 address from a different
interface, you might have grabbed an address from a management interface
that's firewalled away from your transit network. The packets using that
address as a next-hop are going to be silently dropped, which will cause
a blackhole.

A related point: Linux supports assigning the same IPv4 address to multiple
interfaces, so if what you want is sharing a single IPv4 next-hop address
between interfaces, you can already do that.

-- Juliusz