net: dsa: lantiq_gswip: disallow changes to privately set up VID 0

net: dsa: lantiq_gswip: disallow changes to privately set up VID 0

User space can force the altering of VID 0 as it was privately set up by
this driver.

For example, when the port joins a VLAN-aware bridge,
dsa_user_manage_vlan_filtering() will set NETIF_F_HW_VLAN_CTAG_FILTER.
If the port is subsequently brought up and CONFIG_VLAN_8021Q is enabled,
the vlan_vid0_add() function will want to make sure we are capable of
accepting packets tagged with VID 0.

Generally, DSA/switchdev drivers want to suppress that bit of help from
the 8021q layer, and handle VID 0 filters themselves. The 8021q layer
might actually be even detrimential, because VLANs added through
vlan_vid_add() pass through dsa_user_vlan_rx_add_vid(), which is
documented as this:

	/* This API only allows programming tagged, non-PVID VIDs */
	.flags = 0,

so it will force VID 0 to be reconfigured as egress-tagged, non-PVID.
Whereas the driver configures it as PVID and egress-untagged, the exact
opposite.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/9f68340c34b5312c3b8c6c7ecf3cfce574a3f65d.1760566491.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>