bfdd: fix override between sessions

bfdd: fix override between sessions

After two single-hop sessions (*no local address are configured*) on two
interfaces are UP, remove one address of one interface, both of them
(actually, quite independent sessions) come to be DOWN, not just one.

Consider two boxes: A with `a1` and `a2` adddress on two interfaces,
and B with `b1` and `b2`.
Two sessions are set up and ok: `s1` with <a1,b1> and `s2` with <a2,b2>.
After `a1` of A is removed, there is an unhappy coincidence:
1) On A: `s1` changes local address, and sends <a2,b1> packets with help
of route.
2) On B: wrongly regarded <a2,b1> packets with non-zero remote descriminator
as part of `s2`, and are dropped for mismatched remote remote descriminator.
3) On A: `s1` sends <a2,b1> packets with zero remote descriminator to
initialize this session.
4) On B: wrongly regarded <a2,b1> packets with zero remote descriminator as
part of `s2`. Then `s2` will vibrate.

So the good sessions are overridden.

In this case, the <a2,b1> packets with zero remote descriminator won't take
effect until the current good sessions become bad.

Since single-hop sessions are allowed to be set without bound inteface in
current code, this commit adds one check in `bfd_recv_cb()` to avoid wrong
override.

Signed-off-by: anlan_cs <vic.lan@pica8.com>