It's not allowed to create subinterface on a interface that is assigned to VLAN

[johndexter1]

Description

It's not allowed to create subinterface on a interface that is assigned to vlan.

Steps to reproduce the issue:

1. config add vlan 100
2. config vlan member add -u 100 Ethernet0
3. config subinterface add Ethernet0.5 5

Describe the results you received:

You will receive an error "Ethernet0 is configured as a member of vlan. Cannot configure subinterface"

Describe the results you expected:

I would expected subinterface to be created. This is allowed behavior on some platforms, as example, Cumulus Linux.

Output of show version:

202405 branch

I did some testing and managed to achieve what I want (add subinterface on a interface that is member of vlan) and everything is working as expected. This is really useful feature.

To workaround this issue:

1. Create the subinterface first
2. Only then assign interface to vlan