After moving the route control plane code from net/route.c, all rtzone users ended up being in net/route_ctl.c.
Move uma(9) rtzone setup/teardown code to net/route_ctl.c as well to have everything in a single place.
While here, remove custom initializers from the zone. It was added originally to avoid setup/teardown of costy per-cpu couters. With these counters removed, the only remaining job was avoiding rte mutex setup/teardown. Mutex setup is relatively cheap. Additionally, this mutex will soon disappear. With that in mind, there is no sense in keeping custom zone callbacks.