Merge branch 'net-sched-fix-qdisc-graft-hierarchy-validation'

Zijie Huang says:

====================
net/sched: fix qdisc graft hierarchy validation

The qdisc create-and-graft path allows users to keep attaching new classful
qdiscs under an already deep parent hierarchy. Such a hierarchy can later
be walked recursively and exhaust the kernel stack.

This series stores the qdisc hierarchy depth in struct Qdisc and checks it
when a qdisc is grafted. New child qdiscs are rejected once the parent is
already at the maximum allowed depth. It also adds tdc coverage for the
maximum allowed depth and rejection above it.
====================

Link: https://patch.msgid.link/cover.1785434373.git.milkory@outlook.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>