net/sched: taprio: fix NULL pointer dereference in class dump

In the Linux kernel, the following vulnerability has been resolved:

net/sched: taprio: fix NULL pointer dereference in class dump

When a TAPRIO child qdisc is deleted via RTM_DELQDISC, taprio_graft()
is called with new == NULL and stores NULL into q->qdiscs[cl - 1].
Subsequent RTM_GETTCLASS dump operations walk all classes via
taprio_walk() and call taprio_dump_class(), which calls taprio_leaf()
returning the NULL pointer, then dereferences it to read child->handle,
causing a kernel NULL pointer dereference.

The bug is reachable with namespace-scoped CAP_NET_ADMIN on any kernel
with CONFIG_NET_SCH_TAPRIO enabled. On systems with unprivileged user
namespaces enabled, an unprivileged local user can trigger a kernel
panic by creating a taprio qdisc inside a new network namespace,
grafting an explicit child qdisc, deleting it, and requesting a class
dump. The RTM_GETTCLASS dump itself requires no capability.

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000007: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f]
RIP: 0010:taprio_dump_class (net/sched/sch_taprio.c:2478)
Call Trace:
<TASK>
tc_fill_tclass (net/sched/sch_api.c:1966)
qdisc_class_dump (net/sched/sch_api.c:2326)
taprio_walk (net/sched/sch_taprio.c:2514)
tc_dump_tclass_qdisc (net/sched/sch_api.c:2352)
tc_dump_tclass_root (net/sched/sch_api.c:2370)
tc_dump_tclass (net/sched/sch_api.c:2431)
rtnl_dumpit (net/core/rtnetlink.c:6864)
netlink_dump (net/netlink/af_netlink.c:2325)
rtnetlink_rcv_msg (net/core/rtnetlink.c:6959)
netlink_rcv_skb (net/netlink/af_netlink.c:2550)
</TASK>

Fix this by substituting &noop_qdisc when new is NULL in
taprio_graft(), a common pattern used by other qdiscs (e.g.,
multiq_graft()) to ensure the q->qdiscs[] slots are never NULL.
This makes control-plane dump paths safe without requiring individual
NULL checks.

Since the data-plane paths (taprio_enqueue and taprio_dequeue_from_txq)
previously had explicit NULL guards that would drop/skip the packet
cleanly, update those checks to test for &noop_qdisc instead. Without
this, packets would reach taprio_enqueue_one() which increments the root
qdisc's qlen and backlog before calling the child's enqueue; noop_qdisc
drops the packet but those counters are never rolled back, permanently
inflating the root qdisc's statistics.

After this change *old can be a valid qdisc, NULL, or &noop_qdisc.
Only call qdisc_put(*old) in the first case to avoid decreasing
noop_qdisc's refcount, which was never increased.