bgpd: Set extended msg size only if we advertised and received capability

bgpd: Set extended msg size only if we advertised and received capability

If we don't advertise any capabilities (dont-capability-negotiate), we
shouldn't set msg size to 65k only if received this capability from another
peer.

Before:

```
~/frr# vtysh -c 'show ip bgp update-group' | grep 'Max packet size'
    Max packet size: 65535
```

After:

```
~/frr# vtysh -c 'show ip bgp update-group' | grep 'Max packet size'
    Max packet size: 4096
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>