fibs: Suppress the WARNING message for the setups with multiple fibs

[zlei]

Summary

Change 2d3982419593 switched net.add_addr_allfibs default to 0. The
warning message is for potential users of the feature. While since all
supported releases have 0 as default, those potential users may have
already gotten the notification, emitting this WARNING every time
increasing the fib number is less useful but rather confusing to other
users. So let's suppress it now.

MFC after: ???

[zlei]

I'm still thinking if this should be MFCed to stable/14 . I'd prefer to keep stable/13 as is.

[zlei]

I'm planning to land this change at this weekend. @melifaro Any objections ?

[zlei]

In D45971#1047966, @zlei wrote:

I'm still thinking if this should be MFCed to stable/14 . I'd prefer to keep stable/13 as is.

Hi, @markj please have a look at the fix D46204 for stable/14 and stable/13 .