[Bug 285422] IPv4 source address selection is broken (with loopbacks)

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=285422

--- Comment #3 from Lexi Winter <lexi@hemlock.eden.le-fay.org> ---
note that while my example here is a bit contrived, i encountered this problem
in real life in a fairly common scenario:

- OSPFv3 distributes routers' IPv6 loopback addresses
- IPv4 loopback addresses are distributed using IBGP on top of OSPF with BGP
extended next hop
- FreeBSD receives a BGP advertisement for an IPv4 prefix with an IPV6 next hop
- BGP resolves the nexthop of the IPv4 route to a link-local IPv6 address via
OSPF, and adds this route to the kernel FIB
- FreeBSD cannot ping an address on the IPv4 route because it chooses 0.0.0.0
as the source address.

this type of setup (MP-BGP over OSPFv3) is *very* common in modern IP networks,
and it's a significant disadvantage of FreeBSD that it can't operate in this
environment without assigning the /32 loopback to every IP interface.

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