selftests: mlxsw: extack: Test VLAN add on a VXLAN device

Test mapping a VLAN at a VXLAN device that can't be offloaded.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>