networkd: vxlan support setting IPv6 flow labe

networkd: vxlan support setting IPv6 flow labe

This work adds support for setting the IPv6 flow label for vxlan.

vxlan.netdev

NetDev]
Description=vxlan-test
Name=vxlan1
Kind=vxlan

[VXLAN]
Id=33
Local=2405:204:920b:29ac:7e7a:91ff:fe6d:ffe2
Remote=FF02:0:0:0:0:0:1:9
FlowLabel=104
ip -d link show vxlan1

8: vxlan1: <BROADCAST,MULTICAST> mtu 1430 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether be:83:aa:db:6b:cb brd ff:ff:ff:ff:ff:ff promiscuity 0
    vxlan id 33 group ff02::1:9 local 2405:204:920b:29ac:7e7a:91ff:fe6d:ffe2 dev enp0s25 srcport 0 0 dstport 8472 flowlabel 0x68 ageing 300 noudpcsum noudp6zerocsumtx noudp6zerocsumrx addrgenmode eui64 numtxqueues 1 numrxqueues 1