net: bridge: vlan: make __vlan_add_flags react only to PVID and UNTAGGED

Currently there is a very subtle aspect to the behavior of
__vlan_add_flags(): it changes the struct net_bridge_vlan flags and
pvid, yet it returns true ("changed") even if none of those changed,
just a transition of br_vlan_is_brentry(v) took place from false to
true.

This can be seen in br_vlan_add_existing(), however we do not actually
rely on this subtle behavior, since the "if" condition that checks that
the vlan wasn't a brentry before had a useless (until now) assignment:

	*changed = true;

Make things more obvious by actually making __vlan_add_flags() do what's
written on the box, and be more specific about what is actually written
on the box. This is needed because further transformations will be done
to __vlan_add_flags().
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_vlan.c

@@ -58,7 +58,9 @@ static bool __vlan_delete_pvid(struct net_bridge_vlan_group *vg, u16 vid)
 	return true;
 }
 
-/* return true if anything changed, false otherwise */
+/* Returns true if the BRIDGE_VLAN_INFO_PVID and BRIDGE_VLAN_INFO_UNTAGGED bits
+ * of @flags produced any change onto @v, false otherwise
+ */
 static bool __vlan_add_flags(struct net_bridge_vlan *v, u16 flags)
 {
 	struct net_bridge_vlan_group *vg;
@@ -80,7 +82,7 @@ static bool __vlan_add_flags(struct net_bridge_vlan *v, u16 flags)
 	else
 		v->flags &= ~BRIDGE_VLAN_INFO_UNTAGGED;
 
-	return ret || !!(old_flags ^ v->flags);
+	return ret || !!((old_flags ^ v->flags) & BRIDGE_VLAN_INFO_UNTAGGED);
 }
 
 static int __vlan_vid_add(struct net_device *dev, struct net_bridge *br,