net/sched: Abort __tc_modify_qdisc if parent is a clsact/ingress qdisc

Wang reported an illegal configuration [1] where the user attempts to add a
child qdisc to the ingress qdisc as follows:

tc qdisc add dev eth0 handle ffff:0 ingress
tc qdisc add dev eth0 handle ffe0:0 parent ffff:a fq

To solve this, we reject any configuration attempt to add a child qdisc to
ingress or clsact.

[1] https://lore.kernel.org/netdev/[e-mail]/

Fixes: 5e50da01d0ce ("[NET_SCHED]: Fix endless loops (part 2): "simple" qdiscs")
Reported-by: Wang Liang <[e-mail]>
Closes: https://lore.kernel.org/netdev/[e-mail]/
Reviewed-by: Pedro Tammela <[e-mail]>
Acked-by: Jamal Hadi Salim <[e-mail]>
Signed-off-by: Victor Nogueira <[e-mail]>
Reviewed-by: Cong Wang <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/sched/sch_api.c

···
 					NL_SET_ERR_MSG(extack, "Failed to find specified qdisc");
 					return -ENOENT;
 				}
+				if (p->flags & TCQ_F_INGRESS) {
+					NL_SET_ERR_MSG(extack,
+						       "Cannot add children to ingress/clsact qdisc");
+					return -EOPNOTSUPP;
+				}
 				q = qdisc_leaf(p, clid, extack);
 				if (IS_ERR(q))
 					return PTR_ERR(q);