vxlan: metadata based tunneling for IPv6

Support metadata based (formerly flow based) tunneling also for IPv6.
This complements commit ee122c79 ("vxlan: Flow based tunneling").

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>