Merge branch 'revert-tun-tap-vhost-net-apply-qdisc-backpressure-on-full-ptr_ring-to-reduce-tx-drops'

Simon Schippers says:

====================
Revert "tun/tap & vhost-net: apply qdisc backpressure on full ptr_ring to reduce TX drops"

Commit 1d6e569b7d0c ("tun/tap & vhost-net: avoid ptr_ring tail-drop when
a qdisc is present") did not show a relevant performance regression in my
testing, but on Brett Sheffield's librecast testbed it causes a
significant throughput drop in an IPv6 multicast testcase. The regression
can be pinpointed to multiple iperf3 TCP threads sending: for 8 threads
the throughput dropped from 13.5 Gbit/s to 9.13 Gbit/s.

Therefore this series reverts the qdisc backpressure work.

Making the backpressure opt-in via a new IFF_BACKPRESSURE flag was
proposed in [1], but a new IFF_* flag needs more review scrutiny than is
available at the moment, so a revert was requested instead. The opt-in
will be resubmitted for net-next later.

[1] Link: https://lore.kernel.org/netdev/20260709095511.168235-1-simon.schippers@tu-dortmund.de/

Reported-by: Brett Sheffield <brett@librecast.net>
Closes: https://lore.kernel.org/netdev/akVnoOYQOrt8k-Gu@karahi.librecast.net/
====================

Link: https://patch.msgid.link/20260728092240.250257-1-simon.schippers@tu-dortmund.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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