Could you add a NFT_META_BRI_VLAN_PROTO? Similar to patch 1/2, to
retrieve p->br->vlan_proto.

Then, add a generic way to set the vlan metadata. I'm attaching an
incomplete patch, so there is something like:

        meta vlan set 0x88a8:20

to set q-in-q.

we could also add a shortcut for simple vlan case (no q-in-q), ie.
assuming protocol is 0x8100:

        meta vlan set 20

Does this make sense to you?

And we have a way to set the meta vlan information from ingress to
then, which is something I also need here.

Thanks.