MTU is wrong when enabling Wireguard for Multicluster

[tnqn]

Describe the bug

While fixing #5868, @hjiajing and I tested more scenarios and found the issue: when enabling Wireguard for Multicluster, the MTU of all Pod interfaces and wireguard interface were reduced 130 bytes (50 for geneve + 80 for wireguard), however, cross-cluster traffic sent from Pods were not forwarded by wireguard interface.

This is because traffic originated from Pods will be encapsulated on gateway Node, and it's the encapsulated packet which will be encrypted. If the wireguard interface is set with the same MTU as the Pod interface, the encapsulated packet will exceed wireguard interface's MTU.

Versions:

• Antrea version (Docker image tag). 1.14.1 and before

[tnqn]

#5880 will fix this.

cc @luolanzone