fibs: Limit the WARNING message to only once when setting up with multiple fibs

[zlei]

Summary

Suppressing this warning message has been done in main [1] but no plans
to MFC as the message is still useful for users that upgrade from older
releases. Well emitting this warning message every time increasing the
fib number is confusing for users not for the feature net.add_addr_allfibs,
let's limit it to print only once.

1. D45971 fibs: Suppress the WARNING message for the setups with multiple fibs

This is a direct commit to stable/14.

PR: 280097
MFC after: 1 week

[zlei]

This is direct fix for stable/14 and stable/13. The meta commit message

MFC after:

is not accurate. Should be MFS.

[markj]

Seems reasonable to me. In general I'd discourage such changes in stable/14, as it's not a bug fix per se, but this is small and not intrusive.