From the Linux NetDev List:
Running VXLANs over IPv6 link-local addresses allows to use them as a drop-in replacement for VLANs, avoiding to allocate additional outer IP addresses to run the VXLAN over.
The first patch is basically a bugfix, not allowing to use link-local addresses without specifying an interface; it doesn't seem important enough for net/stable though (and without the second patch, allowing to specify link-local addresses at all does not result in a working configuration anyways). The second patch then actually makes VXLAN over link-local IPv6 work by passing interface indices at the right places.
The final patch lifts the limitation of not allowing multiple VXLANs with the same VNI and port, as long as link-local IPv6 addresses are used and different interfaces are specified. Again, this brings VXLAN a bit closer to VLANs feature-wise.