fibs: Suppress the WARNING message for setups with multiple fibs

Change 2d3982419593 switched net.add_addr_allfibs default to 0. The
warning message is for potential users of the feature. Well 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 right now.

PR: 280097
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D45971