feature: support flag configuration for ipvlan

[AryaLee]

Support flag configuration for ipvlan. The value is one of “bridge”, “private”, “vepa”, defaults to “bridge”.

	mv := &netlink.IPVlan{
		LinkAttrs: netlink.LinkAttrs{
			MTU:         conf.MTU,
			Name:        tmpName,
			ParentIndex: m.Attrs().Index,
			Namespace:   netlink.NsFd(int(netns.Fd())),
		},
		Mode: mode,
		Flag: flag,
	}