net: dsa: Give drivers the chance to veto certain upper devices

Some switches rely on unique pvids to ensure port separation in
standalone mode, because they don't have a port forwarding matrix
configurable in hardware. So, setups like a group of 2 uppers with the
same VLAN, swp0.100 and swp1.100, will cause traffic tagged with VLAN
100 to be autonomously forwarded between these switch ports, in spite
of there being no bridge between swp0 and swp1.

These drivers need to prevent this from happening. They need to have
VLAN filtering enabled in standalone mode (so they'll drop frames tagged
with unknown VLANs) and they can only accept an 8021q upper on a port as
long as it isn't installed on any other port too. So give them the
chance to veto bad user requests.

Signed-off-by: Vladimir Oltean <[email protected]>
[Kurt: Pass info instead of ptr]
Signed-off-by: Kurt Kanzenbach <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

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