Both the felix DSA driver and ocelot switchdev driver declare
dev->features & NETIF_F_HW_VLAN_CTAG_FILTER under certain circumstances*,
so the 8021q module will add VID 0 to our RX filter when the port goes
up, to ensure 802.1p traffic is not dropped.

We treat VID 0 as a special value (OCELOT_STANDALONE_PVID) which
deliberately does not have a struct ocelot_bridge_vlan associated with
it. Instead, this gets programmed to the VLAN table in ocelot_vlan_init().

If we allow external calls to modify VID 0, we reach the following
situation:

 # ip link add br0 type bridge vlan_filtering 1 && ip link set br0 up
 # ip link set swp0 master br0
 # ip link set swp0 up # this adds VID 0 to ocelot->vlans with untagged=false
bridge vlan
port              vlan-id
swp0              1 PVID Egress Untagged # the bridge also adds VID 1
br0               1 PVID Egress Untagged
 # bridge vlan add dev swp0 vid 100 untagged
Error: mscc_ocelot_switch_lib: Port with egress-tagged VLANs cannot have more than one egress-untagged (native) VLAN.

This configuration should have been accepted, because
ocelot_port_manage_port_tag() should select OCELOT_PORT_TAG_NATIVE.
Yet it isn't, because we have an entry in ocelot->vlans which says
VID 0 should be egress-tagged, something the hardware can't do.

Fix this by suppressing additions/deletions on VID 0 and managing this
VLAN exclusively using OCELOT_STANDALONE_PVID.

*DSA toggles it when the port becomes VLAN-aware by joining a VLAN-aware
bridge. Ocelot declares it unconditionally for some reason.

Fixes: 54c31984 ("net: mscc: ocelot: enforce FDB isolation when VLAN-unaware")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>