routing: improve debug logging
Use standard logging (FIB_XX_LOG) across nhg code instead of using
old-style DPRINTFs.
Add debug object printer for nhgs (nhgrp_print_buf).
Example:
Jun 19 20:17:09 devel2 kernel: [nhgrp] inet.0 nhgrp_ctl_alloc_default: multipath init done Jun 19 20:17:09 devel2 kernel: [nhg_ctl] inet.0 alloc_nhgrp: num_nhops: 2, compiled_nhop: 2 Jun 19 20:17:26 devel2 kernel: [nhg_ctl] inet.0 alloc_nhgrp: num_nhops: 3, compiled_nhop: 3 Jun 19 20:17:26 devel2 kernel: [nhg_ctl] inet.0 destroy_nhgrp: destroying nhg#0/sz=2:[#6:1,#5:1]
Differential Revision: https://reviews.freebsd.org/D35525
MFC after: 2 weeks