vxlan: fix snooping for link-local IPv6 addresses

If VXLAN is run over link-local IPv6 addresses, it is necessary to store
the ifindex in the FDB entries. Otherwise, the used interface is undefined
and unicast communication will most likely fail.

Support for link-local IPv4 addresses should be possible as well, but as
the semantics aren't as well defined as for IPv6, and there doesn't seem to
be much interest in having the support, it's not implemented for now.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: David S. Miller <davem@davemloft.net>