net: dsa: tag_8021q: Restore bridge VLANs when enabling vlan_filtering

The bridge core assumes that enabling/disabling vlan_filtering will
translate into the simple toggling of a flag for switchdev drivers.

That is clearly not the case for sja1105, which alters the VLAN table
and the pvids in order to obtain port separation in standalone mode.

There are 2 parts to the issue.

First, tag_8021q changes the pvid to a unique per-port rx_vid for frame
identification. But we need to disable tag_8021q when vlan_filtering
kicks in, and at that point, the VLAN configured as pvid will have to be
removed from the filtering table of the ports. With an invalid pvid, the
ports will drop all traffic.  Since the bridge will not call any vlan
operation through switchdev after enabling vlan_filtering, we need to
ensure we're in a functional state ourselves. Hence read the pvid that
the bridge is aware of, and program that into our ports.

Secondly, tag_8021q uses the 1024-3071 range privately in
vlan_filtering=0 mode. Had the user installed one of these VLANs during
a previous vlan_filtering=1 session, then upon the next tag_8021q
cleanup for vlan_filtering to kick in again, VLANs in that range will
get deleted unconditionally, hence breaking user expectation. So when
deleting the VLANs, check if the bridge had knowledge about them, and if
it did, re-apply the settings. Wrap this logic inside a
dsa_8021q_vid_apply helper function to reduce code duplication.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>