Currently, after creating an ingress (or clsact) Qdisc and grafting it
under TC_H_INGRESS (TC_H_CLSACT), it is possible to graft it again under
e.g. a TBF Qdisc:

  $ ip link add ifb0 type ifb
  $ tc qdisc add dev ifb0 handle 1: root tbf rate 20kbit buffer 1600 limit 3000
  $ tc qdisc add dev ifb0 clsact
  $ tc qdisc link dev ifb0 handle ffff: parent 1:1
  $ tc qdisc show dev ifb0
  qdisc tbf 1: root refcnt 2 rate 20Kbit burst 1600b lat 560.0ms
  qdisc clsact ffff: parent ffff:fff1 refcnt 2
                                      ^^^^^^^^

clsact's refcount has increased: it is now grafted under both
TC_H_CLSACT and 1:1.

ingress and clsact Qdiscs should only be used under TC_H_INGRESS
(TC_H_CLSACT).  Prohibit regrafting them.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Fixes: 1f211a1b ("net, sched: add clsact qdisc")
Tested-by: Pedro Tammela <pctammela@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>