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

Test mapping a VLAN at a port device such that on the same VLAN, there
already is an unoffloadable VXLAN device.

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>