net/sched/sch_cake: reduce parent backlog when clearing a tin

The issue:
CAKE has purged unused tins on DiffServ reconfiguration since commit
83f8fd69af4f ("sch_cake: Add DiffServ handling") without propagating the
resulting backlog reduction to classful parents. The issue was noticed
while reviewing 7cbfb180945c, but predates that change.

cake_clear_tin() drops every packet from a tin with
kfree_skb_reason() but never calls qdisc_tree_reduce_backlog(), so a
CAKE qdisc under a classful parent (e.g. HTB) leaves the dropped packets
in the parent qdisc's and class's qlen/backlog accounting forever. The
normal dequeue path does reduce the parent; only the live reconfigure
purge misses it.

The fix:
Tins are cleared from cake_reset() and, on reconfigure, from
cake_configure_rates(). The qdisc core already owns the parent delta for
the reset path: qdisc_purge_queue() snapshots qlen/backlog, runs
qdisc_reset(), then reduces the tree, so propagating the reduction from
cake_clear_tin() itself would subtract the same backlog twice. Snapshot
CAKE's qlen/backlog around only the reconfiguration purge in
cake_configure_rates() and reduce the parent once afterwards, matching
the delta pattern already used by cake_enqueue().

Conditions to recreate the bug:
put CAKE (diffserv8) under an HTB class, flood DSCP-classified traffic
into a high tin so it backlogs, then change CAKE to diffserv3. The HTB
parent keeps the purged packets in its qlen and backlog; a kernel with
the fix shows the parent at the leaf's backlog (426b/5p) instead of
retaining the purged packets.

Reported-by: Sashiko (gemini) <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260816012109.2865223-1-ooonea@gmail.com
Link: https://lore.kernel.org/netdev/20260816012109.2865223-1-ooonea@gmail.com/
Link: https://lore.kernel.org/netdev/8733ugtry8.fsf@toke.dk/
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/QDISC-5RTM.v3.r0.20261020053711-2@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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