Add new rt_foreach_fib_walk_del() function for deleting route entries

by filter function instead of picking into routing table details in
each consumer.

Remove now-unused rt_expunge() (eliminating last external RTF_RNH_LOCKED
user).
This simplifies future nexthops/mulitipath changes and rtrequest1_fib()

locking refactoring.

Actual changes:
Add "rt_chain" field to permit rte grouping while doing batched delete

from routing table (thus growing rte 200->208 on amd64).

Add "rti_filter" / "rti_filterdata" / "rti_spare" fields to rt_addrinfo

to pass filter function to various routing subsystems in standard way.

Convert all rt_expunge() customers to new rt_addinfo-based api and eliminate

rt_expunge().