Refactor rib iterator functions.

• Make rib_walk() order of arguments consistent with the rest of RIB api
• Add rib_walk_ext() allowing to exec callback before/after iteration.
• Rename rt_foreach_fib_walk_del -> rib_foreach_table_walk_del
• Rename rt_forach_fib_walk -> rib_foreach_table_walk
• Move rib_foreach_table_walk{_del} to route/route_helpers.c
• Slightly refactor rib_foreach_table_walk{_del} to make the implementation consistent and prepare for upcoming iterator optimizations.

Differential Revision: https://reviews.freebsd.org/D27219