Merge branch 'big-tcp-without-hbh-in-ipv6'

Merge branch 'big-tcp-without-hbh-in-ipv6'

Alice Mikityanska says:

====================
BIG TCP without HBH in IPv6

Resubmitting after the grace period.

This series is part 1 of "BIG TCP for UDP tunnels". Due to the number of
patches, I'm splitting it into two logical parts:

* Remove hop-by-hop header for BIG TCP IPv6 to align with BIG TCP IPv4.
* Fix up things that prevent BIG TCP from working with UDP tunnels.

The current BIG TCP IPv6 code inserts a hop-by-hop extension header with
32-bit length of the packet. When the packet is encapsulated, and either
the outer or the inner protocol is IPv6, or both are IPv6, there will be
1 or 2 HBH headers that need to be dealt with. The issues that arise:

1. The drivers don't strip it, and they'd all need to know the structure
of each tunnel protocol in order to strip it correctly, also taking into
account all combinations of IPv4/IPv6 inner/outer protocols.

2. Even if (1) is implemented, it would be an additional performance
penalty per aggregated packet.

3. The skb_gso_validate_network_len check is skipped in
ip6_finish_output_gso when IP6SKB_FAKEJUMBO is set, but it seems that it
would make sense to do the actual validation, just taking into account
the length of the HBH header. When the support for tunnels is added, it
becomes trickier, because there may be one or two HBH headers, depending
on whether it's IPv6 in IPv6 or not.

At the same time, having an HBH header to store the 32-bit length is not
strictly necessary, as BIG TCP IPv4 doesn't do anything like this and
just restores the length from skb->len. The same thing can be done for
BIG TCP IPv6. Removing HBH from BIG TCP would allow to simplify the
implementation significantly, and align it with BIG TCP IPv4, which has
been a long-standing goal.
====================

Link: https://patch.msgid.link/20260205133925.526371-1-alice.kernel@fastmail.im
Signed-off-by: Jakub Kicinski <kuba@kernel.org>