(Part of a larger [[ https://reviews.freebsd.org/differential/revision/edit/22988/ | D22988 ]] ).
Redirect (and temporal) route expiration was broken a while ago.
This change brings route expiration back, with unified IPv4/IPv6 handling code.
It introduces `net.inet.icmp.redirtimeout` sysctl, allowing to set an expiration time for redirected routes. It defaults to 10 minutes, analogues with `net.inet6.icmp6.redirtimeout`.
Implementation uses separate file, `route_temporal.c`, as `route.c` is already bloated with tons of different functions.
Internally, expiration is implemented as an per-rnh callout scheduled when route with non-zero rt_expire time is added or rt_expire is changed. It does not add any overhead when no temporal routes are present.
Callout traverses entire routing tree under wlock, scheduling expired routes for deletion and calculating the next time it needs to be run.
The rationale for such implemention is the following: typically workloads requiring large amount of routes have redirects turned off already, while the systems with small amount of routes will not inhibit large overhead during tree traversal.
To support this callout, `rib_fibnum`, `rib_family` and `rib_vnet` fields are added to the rhn. These will be used by the upcoming route changes as well.
This changes also fixes `netstat -rn` display of route expiration time, which has been broken since the conversion from kread() to sysctl.