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
```