net: dsa: sja1105: use 4095 as the private VLAN for untagged traffic

One thing became visible when writing the blamed commit, and that was
that STP and PTP frames injected by net/dsa/tag_sja1105.c using the
deferred xmit mechanism are always classified to the pvid of the CPU
port, regardless of whatever VLAN there might be in these packets.

So a decision needed to be taken regarding the mechanism through which
we should ensure that delivery of STP and PTP traffic is possible when
we are in a VLAN awareness mode that involves tag_8021q. This is because
tag_8021q is not concerned with managing the pvid of the CPU port, since
as far as tag_8021q is concerned, no traffic should be sent as untagged
from the CPU port. So we end up not actually having a pvid on the CPU
port if we only listen to tag_8021q, and unless we do something about it.

The decision taken at the time was to keep VLAN 1 in the list of
priv->dsa_8021q_vlans, and make it a pvid of the CPU port. This ensures
that STP and PTP frames can always be sent to the outside world.

However there is a problem. If we do the following while we are in
the best_effort_vlan_filtering=true mode:

ip link add br0 type bridge vlan_filtering 1
ip link set swp2 master br0
bridge vlan del dev swp2 vid 1

Then untagged and pvid-tagged frames should be dropped. But we observe
that they aren't, and this is because of the precaution we took that VID
1 is always installed on all ports.

So clearly VLAN 1 is not good for this purpose. What about VLAN 0?
Well, VLAN 0 is managed by the 8021q module, and that module wants to
ensure that 802.1p tagged frames are always received by a port, and are
always transmitted as VLAN-tagged (with VLAN ID 0). Whereas we want our
STP and PTP frames to be untagged if the stack sent them as untagged -
we don't want the driver to just decide out of the blue that it adds
VID 0 to some packets.

So what to do?

Well, there is one other VLAN that is reserved, and that is 4095:
$ ip link add link swp2 name swp2.4095 type vlan id 4095
Error: 8021q: Invalid VLAN id.
$ bridge vlan add dev swp2 vid 4095
Error: bridge: Vlan id is invalid.

After we made this change, VLAN 1 is indeed forwarded and/or dropped
according to the bridge VLAN table, there are no further alterations
done by the sja1105 driver.

Fixes: ec5ae610

 ("net: dsa: sja1105: save/restore VLANs using a delta commit method")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>