Merge branch 'net-sched-refactor-qdisc-drop-reasons-into-dedicated-tracepoint'

Jesper Dangaard Brouer says:

====================
net: sched: refactor qdisc drop reasons into dedicated tracepoint

This series refactors qdisc drop reason handling by introducing a dedicated
enum qdisc_drop_reason and trace_qdisc_drop tracepoint, providing qdisc
layer drop diagnostics with direct qdisc context visibility.

Background:
-----------
Identifying which qdisc dropped a packet via skb_drop_reason is difficult.
Normally, the kfree_skb tracepoint caller "location" hints at the dropping
code, but qdisc drops happen at a central point (__dev_queue_xmit), making
this unusable. As a workaround, commits 5765c7f6e317 ("net_sched: sch_fq:
add three drop_reason") and a42d71e322a8 ("net_sched: sch_cake: Add drop
reasons") encoded qdisc names directly in the drop reason enums.

This series provides a cleaner solution by creating a dedicated qdisc
tracepoint that naturally includes qdisc context (handle, parent, kind).

Solution:
---------
Create a new tracepoint trace_qdisc_drop that builds on top of existing
trace_qdisc_enqueue infrastructure. It includes qdisc handle, parent,
qdisc kind (name), and device information directly.

The existing SKB_DROP_REASON_QDISC_DROP is retained for backwards
compatibility via kfree_skb_reason(). The qdisc-specific drop reasons
(QDISC_DROP_*) provide fine-grained detail via the new tracepoint.
The enum uses subsystem encoding (offset by SKB_DROP_REASON_SUBSYS_QDISC)
to catch type mismatches during debugging.

This implements the alternative approach described in:
https://lore.kernel.org/all/6be17a08-f8aa-4f91-9bd0-d9e1f0a92d90@kernel.org/
====================

Link: https://patch.msgid.link/177211325634.3011628.9343837509740374154.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>