vxlan: Enable MDB support

Now that the VXLAN MDB control and data paths are in place we can expose
the VXLAN MDB functionality to user space.

Set the VXLAN MDB net device operations to the appropriate functions,
thereby allowing the rtnetlink code to reach the VXLAN driver.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>