router fatal netlink error if kernel does not support openvswitch vxlan vports

[dpw]

See https://groups.google.com/a/weave.works/forum/#!topic/weave-users/vSn7vizptt8

You can reproduce this on any distro by doing:

• rmmod vport_vxlan
• Temporarily doing mv /lib/modules/<kernel version>/kernel/net/openvswitch/vport-vxlan.ko{,.disable}