Merge branch 'net_sched-make-qlen_notify-idempotent'

Cong Wang says:

====================
net_sched: make ->qlen_notify() idempotent

Gerrard reported a vulnerability exists in fq_codel where manipulating
the MTU can cause codel_dequeue() to drop all packets. The parent qdisc's
sch->q.qlen is only updated via ->qlen_notify() if the fq_codel queue
remains non-empty after the drops. This discrepancy in qlen between
fq_codel and its parent can lead to a use-after-free condition.

Let's fix this by making all existing ->qlen_notify() idempotent so that
the sch->q.qlen check will be no longer necessary.

Patch 1~5 make all existing ->qlen_notify() idempotent to prepare for
patch 6 which removes the sch->q.qlen check. They are followed by 5
selftests for each type of Qdisc's we touch here.

All existing and new Qdisc selftests pass after this patchset.

Fixes: 4b549a2ef4be ("fq_codel: Fair Queue Codel AQM")
Fixes: 76e3cc126bb2 ("codel: Controlled Delay AQM")
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
====================

Link: https://patch.msgid.link/20250403211033.166059-1-xiyou.wangcong@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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