How to make ospf advertise stubnet base on the interface/subnet state?

[chan alfie]

Hi, community

I have create a loopback interface on linux and assign 4 subnets on it.

```
ip link add dev lo235 type dummy
ip link set lo235 up
ip addr add 10.23.5.1/30 dev lo235
ip addr add 10.23.5.5/30 dev lo235
ip addr add 10.23.5.9/30 dev lo235
ip addr add 10.23.5.13/30 dev lo235
```

stubnet <prefix> { summary;}; is used on an originator routers(r235) ospf area, 
and it indeed propagted the summary prefix instead of subnets, here is the 
config:

r235:
```
area 10.23.0.0 {
  ...
  stubnet 10.23.5.0/28 {
    summary;
  }
  ...
}
```

r235:
```
show ospf state
...
        router 10.23.0.5
                distance 0
                ...
                stubnet 10.23.5.0/28 metric 10
...
```

but when I shutdown the lo235 or delete all of the subnets, the 10.23.5.0/28 is 
still advertised by r235.

r235
```
ip link set lo235 down
or
ip addr del 10.23.5.1/30 dev lo235
ip addr del 10.23.5.5/30 dev lo235
ip addr del 10.23.5.9/30 dev lo235
ip addr del 10.23.5.13/30 dev lo235
```

r235
```
bird> show ospf state
        router 10.23.0.5
                distance 0
                ...
                stubnet 10.23.5.0/28 metric 10
```

How to make ospf advertise stubnet base on the interface/subnet state, 
(interface up/down, subnets existed or not)?

By the way, networks{} clause works as expected. When the subnet of originator 
router is dismiss, the ABR's networks will take effect(disappear of appear).