selftests: mlxsw: extack: Test bridge creation with VXLAN

Test that creation of a bridge (both VLAN-aware and VLAN-unaware) fails
with an extack when a VXLAN device with an unsupported configuration is
already enslaved to it.

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