Merge branch 'net-sched-sch_cake-fixes-for-accounting-and-reconfiguration'

Jamal Hadi Salim says:

====================
net/sched: sch_cake: fixes for accounting and reconfiguration

Four independent fixes in the CAKE qdisc.  Three are follow-ups to the
autorate reconfiguration work around commit 7cbfb180945c ("net/sched:
sch_cake: fix autorate reconfiguration throttling") and the 32-bit
admission-counter wraps fixed in gred/bfifo/plug by commit 4c660ee8c809
("net: sched: fix 32-bit backlog wrap in gred, bfifo and plug enqueue");
the fourth is a dead-code cleanup found while auditing the dump path.

Patch 1 serializes a 'tc qdisc change' with the datapath's autorate
update under the qdisc lock, so the netlink writer and the autorate
writer can no longer lose the configured rate.  rate_bps needs no atomic
(it is only loaded and stored), so it stays a plain u64 with
READ_ONCE()/WRITE_ONCE() annotations.

Patch 2 propagates the backlog reduction of the reconfigure-time tin
purge to classful parents, so a CAKE leaf under HTB no longer leaves the
purged packets in the parent's qlen/backlog forever.

Patch 3 widens buffer_used and buffer_max_used to u64.  The u32
accumulator wraps once the resident truesize passes 2^32, bypassing the
over-limit drop loop while the configured limit is a u32 attribute; the
u32 MEMORY_USED export is clamped so widening the counter does not
truncate the reported value.

Patch 4 drops an unreachable 'nla_nest_end() < 0' error check: the
helper never returns a negative value.

All four are hardening for net-next (no Fixes:, no Cc: stable): the
counter wrap, the missing parent-backlog propagation and the
reconfiguration race all predate any contract we could anchor a
regression on.  Patches 1-2 were reported by Sashiko (gemini) and patch
3 comes from the gred 32-bit-wrap family; patch 4 is a static cleanup.
====================

Link: https://patch.msgid.link/QDISC-5RTM.v3.r0.20261020053711@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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