routing: convert remnants of DPRINTF to FIB_CTL_LOG().
Convert the last remaining pieces of old-style debug messages
to the new debugging framework.
Differential Revision: https://reviews.freebsd.org/D35994
MFC after: 2 weeks
(cherry picked from commit 5c23343b8c0cc018f06a4a2cb2d911770bdc8ee2)