Add rib_walk_from() wrapper for selective rib tree traversal.

Provide wrapper for the rnh_walktree_from() rib callback.
As currently struct rib_head is considered internal to the
routing subsystem, this wrapper is necessary to maintain isolation
from the external code.

Differential Revision: https://reviews.freebsd.org/D29971
MFC after: 1 week

(cherry picked from commit f9668e42b44f22c9ab213eb9a4792d5acfcf6903)