Convert debugnet to the new routing KPI.

Introduce new fib[46]_lookup_debugnet() functions serving as a
special interface for the crash-time operations. Underlying
implementation will try to return lookup result if
datastructures are not corrupted, avoding locking.

Convert debugnet to use fib4_lookup_debugnet() and switch it
to use nexthops instead of rtentries.

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D24555