Move rtzone handling code to net/route_ctl.c

Move rtzone handling code to net/route_ctl.c

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 be removed. With that in mind, there is no sense in keeping
 custom zone callbacks.

Differential Revision:	https://reviews.freebsd.org/D26051