Describe the bug
TestNetworkPolicyMetrics failed on an irrelevant PR vmware-tanzu/antrea#1389, and race was detected:

2020-10-15T09:14:50.3969191Z --- FAIL: TestNetworkPolicyMetrics (0.05s)
2020-10-15T09:14:50.3989561Z     networkpolicy_controller_test.go:528: 
2020-10-15T09:14:50.3990066Z         	Error Trace:	networkpolicy_controller_test.go:528
2020-10-15

What

It is handy to configure the MTU of veth links automatically.

c.f. https://itnext.io/benchmark-results-of-kubernetes-network-plugins-cni-over-10gbit-s-network-updated-august-2020-6e1b757b9e49

How

In coild,

1. List all physical Device links with up status: https://pkg.go.dev/github.com/vishvananda/netlink#Device
2. Choose the minimum MTU among the links
3. Create v