net/sched: fix lockdep splat in qdisc_tree_reduce_backlog()

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

net/sched: fix lockdep splat in qdisc_tree_reduce_backlog()

qdisc_tree_reduce_backlog() is called with the qdisc lock held,
not RTNL.

We must use qdisc_lookup_rcu() instead of qdisc_lookup()

syzbot reported:

WARNING: suspicious RCU usage
6.1.74-syzkaller #0 Not tainted

net/sched/sch_api.c:305 suspicious rcu_dereference_protected() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
3 locks held by udevd/1142:
#0: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:306 [inline]
#0: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:747 [inline]
#0: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: net_tx_action+0x64a/0x970 net/core/dev.c:5282
#1: ffff888171861108 (&sch->q.lock){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:350 [inline]
#1: ffff888171861108 (&sch->q.lock){+.-.}-{2:2}, at: net_tx_action+0x754/0x970 net/core/dev.c:5297
#2: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:306 [inline]
#2: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:747 [inline]
#2: ffffffff87c729a0 (rcu_read_lock){....}-{1:2}, at: qdisc_tree_reduce_backlog+0x84/0x580 net/sched/sch_api.c:792

stack backtrace:
CPU: 1 PID: 1142 Comm: udevd Not tainted 6.1.74-syzkaller #0
Har…