Allow VRFs for FPM

[thorvald]

Right now, all VRFs other than 0 are filtered from FPM. Yet the code already exists in the encoding to transport the table-id in the messages. As such, a surgical fix to zfpm_is_table_for_fpm is all that's needed to export VRFs.

We've done a straight patch for this in our own code, but we'd like to upstream the change. We see three solution paths:

1. Always enable VRFs to be sent; this changes existing behavior.
2. Introduce a config option for no fpm filter vrfs; this opens the question of what happens to an already-open FPM connection if this option is changed.
3. Extend the module init syntax, so that you specify -M fpm:netlink:1 or similar to enable unfiltered VRFs.

Option 3 seems the cleanest, but which option has the highest probability of PR acceptance?

[donaldsharp]

From FRR's perspective turning on the passing of table-id's is a no-brainer. Would not passing down the vrf routes be there to protect the FPM listener? The FPM listener should be paying attention to the passed table-id. So I see no reason to just pass the vrf tableid(option #1)

[thorvald]

Just to make sure I understand, you're saying you are in favor of option #1? That's certainly the easiest to implement :)

[donaldsharp]

Yes exactly