debugnet: remove spurious message on boot

In non-INVARIANTS kernels, hide the warning message printed by debugnet
when an interface MTU is configured or link state changes, and debugnet
cannot infer the number of mbuf clusters to reserve. The warning isn't
really actionable and mostly serves to confuse users.

Reviewed by: vangyzen, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34393