networkd: VXLAN netdev does not assign group address

[fooker]

systemd version the issue has been seen with
systemd 243 (v243.8-1.fc31)

Used distribution
Fedora 31 (Server Edition)

Expected behaviour you didn't see
After creating a VXLAN netdev with a Group multicast address, the group address should be shown in ip -d link show NETDEV and networkctl show NETDEV.

Unexpected behaviour you saw
The group address is not assigned to the interface. No error is shown in debug log.

Steps to reproduce the problem
Create a VXLAN netdev (and a simple network) unit containing the following:

[NetDev]
Kind=vxlan
Name=myvx

[VXLAN]
VNI=1
Group=ff02::42:1
DestinationPort=8472

After restarting systemd-networkd the VXLAN device is created, but the group address is not assigned. Use ip -d link show myvx and networkctl show myvx to verify.

In contrast, creating the device using iproute2 (ip -6 link add name myvx type vxlan id 1 dev enp8s0 dstport 8472 group ff02::42:1) will assign the group address and it will show up in both commands.

The logs don't show any error or warning.

[ssahani]

Fix is #15397