bgpd: Warn user only if the LL is not seriously available

bgpd: Warn user only if the LL is not seriously available

LL address is assigned, but we get a warning, that it's not:

Interface: enp3s0 does not have a v6 LL address associated with it, waiting until one is created for it

```
donatas-pc# sh int enp3s0
Interface enp3s0 is up, line protocol is up
  Link ups:       0    last: (never)
  Link downs:     0    last: (never)
  vrf: default
  index 2 metric 0 mtu 1500 speed 100
  flags: <UP,BROADCAST,RUNNING,MULTICAST>
  v4 Multicast forwarding is on
  v6 Multicast forwarding is on
  Type: Ethernet
  HWaddr: 18:c0:4d:96:fa:3f
  inet 192.168.10.17/24
  inet6 2a02:4780:abc:0:e776:6220:1e21:44b1/64
  inet6 fe80::ca5d:fd0d:cd8:1bb7/64
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>