net/sched: sch_clsact: Only create under TC_H_CLSACT

clsact Qdiscs are only supposed to be created under TC_H_CLSACT (which
equals TC_H_INGRESS).  Return -EOPNOTSUPP if 'parent' is not
TC_H_CLSACT.

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>