[RFC] gtp: add 5G PDU Session Container (QFI) support?

Hi Pablo, Harald,

The in-tree GTP driver supports GTPv0 and GTPv1-U, but has no handling
for the 5G N3 PDU Session Container (PSC) extension header (type 0x85,
3GPP TS 29.281 / TS 38.415) that carries the QoS Flow Identifier (QFI).
drivers/net/gtp.c also still has the "TODO: Support for extension
header, sequence number and N-PDU" on the xmit path.

I'd like to add PSC/QFI support, but before writing the datapath code I
wanted to check two things with you.

1) Is this something you'd welcome in mainline gtp.c?  I'm aware a lot of
   5G user-plane work lives in out-of-tree datapaths (gtp5g, the eBPF
   eUPF), so I don't want to add datapath complexity you'd rather not
   carry here.

2) If yes: on RX, once the QFI has been parsed out of the PSC, what would
   you like the driver to do with it?  The options I see are:
     - stash it in skb->mark, so tc/nftables can classify uplink traffic
       by QoS flow;
     - attach it as skb metadata (tc_skb_ext / metadata_dst); or
     - just validate it and otherwise ignore it.
   The shape of the RX patch depends on this, so I'd rather ask than
   guess.

The plan would be a small incremental series:
  1. add a GTPA_QFI (u8) netlink attribute + ynl spec, stored per PDP
     context (control-plane plumbing only);
  2. RX: parse the PSC extension header and extract the QFI;
  3. TX: build the PSC from the per-PDP QFI on the downlink path (the
     existing extension-header TODO).

Patch 1 is already written and builds; I'll post the series once I know
whether you want it in mainline and how you'd prefer the RX QFI to be
surfaced.  Happy to adjust the approach.

Thanks,
Anil