Backport request: fq_codel stale-state fix (815980fe6dbb)

[Gyutae Bae <gyutae.opensource@navercorp.com>]

Hi,

Please backport the following upstream commit to 5.10.y, 5.15.y,
6.1.y, 6.6.y, 6.12.y and 6.18.y:

  815980fe6dbb01ad4007e8b260a45617f598b76d
  net_sched: codel: fix stale state for empty flows in fq_codel

This fixes stale CoDel state that causes packet drops in sparse
flows.  When a flow's queue becomes empty, first_above_time is not
reset, so a later packet can be dropped based on the old timestamp.

We have encountered this in production on 5.15 nodes using the default
mq + fq_codel setup, with steady qdisc drops on lightly loaded links.
A kprobe on the fq_codel drop path showed that every drop we traced
occurred with first_above_time set more than one second earlier.
Applying the fix as a livepatch on a 5.15 node reduced its drops from
about 50 per minute to zero.  We also confirmed that the fix eliminated
these drops in tests that reproduced the issue on 5.15.92 and 6.12.48.

The fix adds one line to include/net/codel_impl.h.  It applies cleanly
to v5.10.272, v5.15.223, v6.1.190, v6.6.160, v6.12.114 and v6.18.58.
Both commits identified by the Fixes: tags are present in all six
longterm trees.

The commit was merged in v7.0 without a Cc: stable tag and was
backported to 6.19.11 as 86b81a5c24ad.  It is already included in
7.2.y, but is still missing from the longterm trees listed above.

Thanks,
Gyutae Bae