net: dsa: tag_8021q: Create a stable binary format

Tools like tcpdump need to be able to decode the significance of fake
VLAN headers that DSA uses to separate switch ports.

But currently these have no global significance - they are simply an
ordered list of DSA_MAX_SWITCHES x DSA_MAX_PORTS numbers ending at 4095.

The reason why this is submitted as a fix is that the existing mapping
of VIDs should not enter into a stable kernel, so we can pretend that
only the new format exists. This way tcpdump won't need to try to make
something out of the VLAN tags on 5.2 kernels.

Fixes: f9bbe447

 ("net: dsa: Optional VLAN-based port separation for switches without tagging")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>