Files in /proc/net/vlan are being set to 0600 permissions

[Daniel Amaya]

A fresh install of Ubuntu 20.04. Installed MAAS regiond controller. Here's Netplan config:

```
network:
  ethernets:
    enp4s0f0: {}
  vlans:
    vlan3:
      id: 3
      link: enp4s0f0
      addresses: [10.3.9.24/16]
      gateway4: 10.3.0.1
      mtu: 9000
      nameservers:
          addresses: [10.3.0.1, 10.3.0.2]
  version: 2
```

Here's permissions for files created in `/proc/net/vlan`

```
root@d9-33u-a:~# ls -lh /proc/net/vlan/
total 0
-rw------- 1 root root 0 May 25 04:33 config
-rw------- 1 root root 0 May 25 04:33 vlan3
```

This is causing issues with MAAS v3.0 beta:

```
2021-05-24 23:27:36 provisioningserver.rpc.clusterservice: [critical] Event-loop 'd9-34u-b:pid=108335' handshake failed; dropping connection.
        Traceback (most recent call last):
          File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
            result = g.send(result)
          File "/usr/lib/python3/dist-packages/provisioningserver/rpc/clusterservice.py", line 1176, in performHandshake
            registered = yield self.registerRackWithRegion()
          File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1613, in unwindGenerator
            return _cancellableInlineCallbacks(gen)
          File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1529, in _cancellableInlineCallbacks
            _inlineCallbacks(None, g, status)
        --- <exception caught here> ---
          File "/usr/lib/python3/dist-packages/provisioningserver/rpc/clusterservice.py", line 1176, in performHandshake
            registered = yield self.registerRackWithRegion()
          File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
            result = g.send(result)
          File "/usr/lib/python3/dist-packages/provisioningserver/rpc/clusterservice.py", line 1123, in registerRackWithRegion
            interfaces = get_all_interfaces_definition()
          File "/usr/lib/python3/dist-packages/provisioningserver/utils/network.py", line 1133, in get_all_interfaces_definition
            for name, ipaddr in get_ip_addr().items()
          File "/usr/lib/python3/dist-packages/provisioningserver/utils/ipaddr.py", line 43, in get_ip_addr
            output = call_and_check([get_resources_bin_path()])
          File "/usr/lib/python3/dist-packages/provisioningserver/utils/shell.py", line 106, in call_and_check
            raise ExternalProcessError(process.returncode, command, output=stderr)
        provisioningserver.utils.shell.ExternalProcessError: Command `/usr/share/maas/machine-resources/amd64` returned non-zero exit status 1:
        ERROR: open /proc/net/vlan/config: permission denied
```

I can modify the permissions manually, which fixes things; but a reboot will set permissions back to 0600.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.