net: dsa: sja1105: Fix broken learning with vlan_filtering disabled

When put under a bridge with vlan_filtering 0, the SJA1105 ports will
flood all traffic as if learning was broken. This is because learning
interferes with the rx_vid's configured by dsa_8021q as unique pvid's.

So learning technically still *does* work, it's just that the learnt
entries never get matched due to their unique VLAN ID.

The setting that saves the day is Shared VLAN Learning, which on this
switch family works exactly as desired: VLAN tagging still works
(untagged traffic gets the correct pvid) and FDB entries are still
populated with the correct contents including VID. Also, a frame cannot
violate the forwarding domain restrictions enforced by its classified
VLAN. It is just that the VID is ignored when looking up the FDB for
taking a forwarding decision (selecting the egress port).

This patch activates SVL, and the result is that frames with a learnt
DMAC are no longer flooded in the scenario described above.

Now exactly *because* SVL works as desired, we have to revisit some
earlier patches:

- It is no longer necessary to manipulate the VID of the 'bridge fdb
  {add,del}' command when vlan_filtering is off. This is because now,
  SVL is enabled for that case, so the actual VID does not matter*.

- It is still desirable to hide dsa_8021q VID's in the FDB dump
  callback. But right now the dump callback should no longer hide
  duplicates (one per each front panel port's pvid, plus one for the
  VLAN that the CPU port is going to tag a TX frame with), because there
  shouldn't be any (the switch will match a single FDB entry no matter
  its VID anyway).

* Not really... It's no longer necessary to transform a 'bridge fdb add'
  into 5 fdb add operations, but the user might still add a fdb entry with
  any vid, and all of them would appear as duplicates in 'bridge fdb
  show'. So force a 'bridge fdb add' to insert the VID of 0**, so that we
  can prune the duplicates at insertion time.

** The VID of 0 is better than 1 because it is always guaranteed to be
   in the ports' hardware filter. DSA also avoids putting the VID inside
   the netlink response message towards the bridge driver when we return
   this particular VID, which makes it suitable for FDB entries learnt
   with vlan_filtering off.

Fixes: 227d07a0 ("net: dsa: sja1105: Add support for traffic through standalone ports")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Georg Waibel <georg.waibel@sensor-technik.de>
Signed-off-by: David S. Miller <davem@davemloft.net>