Flush remaining routes from the routing table during VNET shutdown.

[melifaro]

Summary

This fixes rtentry leak for the cloned interfaces created inside the
VNET.

PR: 253998
Reported by: rashey at superbox.pl
MFC after: 3 days

Current code cleans up protocol state in ip[6]_destroy(). It includes flushing route entries on a per-interface basis using rt_flushifroutes_af(). The latter function removes all interface routes except ones with RTF_PINNED flag (e.g. interface routes).
As a result, some of the rtentries may leak.

Add rib_flush_routes_af() which removes all of the routes for the specified family in a light-weight fashion and switch IPv4/IPv6 stack teardown to use it instead of iterating over all interfaces with rt_flushroutes_af().

Test Plan

# kyua test -k /usr/tests/sys/netpfil/pf/Kyuafile set_skip:set_skip_group_lo
set_skip:set_skip_group_lo  ->  passed  [0.053s]
tail -n 200 /var/log/messages | grep rtentry
#

[melifaro]

Implement a cleaner fix.