ipvlan: forbid vlan devices on top of ipvlan

Currently we allow the creation of 8021q devices on top of
ipvlan, but such devices are nonfunctional, as the underlying
ipvlan rx_hanlder hook can't match the relevant traffic.

Be explicit and forbid the creation of such nonfunctional devices.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>