bfdd: disable echo socket when not using it

[rzalamena]

Fixes #16594.

Lets avoid a performance penalty in forwarding when not using the BFD echo feature. The echo socket uses raw packet capturing along with a BPF filter which causes performance issues.

[donaldsharp]

once ci finishes I'll get this in.

[riw777]

looks like closing this socket is causing bfdd not to be able to "find itself" ... the test probably needs to be changed (?)

[RodrigoMNardi]

ci:rerun

[crosser]

I can confirm that the change helps. I backported the commit to 10.1.1. With running bfdd, creating 250 VRFs does not produce noticeable impact on the network throughput.

Thanks @rzalamena !

[riw777]

It's odd it's only failing on docker ... let's try again

ci:rerun

[riw777]

I think the CI test needs to be looked at ... probably not going to pass ...