bgpd: Show max packet size per update-group

bgpd: Show max packet size per update-group

```
exit1-debian-9# sh ip bgp update-groups
Update-group 2:
  Created: Fri Jul  2 20:47:23 2021
  MRAI value (seconds): 0

  Update-subgroup 2:
    Created: Fri Jul  2 20:47:23 2021
    Join events: 1
    Prune events: 0
    Merge events: 0
    Split events: 0
    Update group switch events: 0
    Peer refreshes combined: 0
    Merge checks triggered: 0
    Coalesce Time: 1200
    Version: 14
    Packet queue length: 0
    Total packets enqueued: 8
    Packet queue high watermark: 1
    Adj-out list count: 11
    Advertise list: empty
    Flags:
    Max packet size: 65535
    Peers:
      - eth1
Update-group 3:
  Created: Fri Jul  2 20:47:43 2021
  MRAI value (seconds): 0

  Update-subgroup 3:
    Created: Fri Jul  2 20:47:43 2021
    Join events: 1
    Prune events: 0
    Merge events: 0
    Split events: 0
    Update group switch events: 0
    Peer refreshes combined: 0
    Merge checks triggered: 0
    Coalesce Time: 1200
    Version: 14
    Packet queue length: 0
    Total packets enqueued: 8
    Packet queue high watermark: 1
    Adj-out list count: 11
    Advertise list: empty
    Flags:
    Max packet size: 4096
    Peers:
      - 192.168.10.17
```

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