physical: Improve treatment of localnet non-VLAN logical ports.

physical: Improve treatment of localnet non-VLAN logical ports.

Until now, the flow table treated localnet logical ports that have a VLAN
quite differently from those that don't.  The ones without a VLAN were
essentially trunk ports: any packets that came in, that weren't picked off
by a localnet port with a VLAN, were passed to the ones without a VLAN.
This wasn't the intended behavior.

This commit changes behavior to the intended behavior.  Now, localnet ports
without a specific VLAN only receive packets without a VLAN header or those
with VLAN ID 0 (with that header stripped off).

Found by inspection.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <russell@ovn.org>