net/sched: fix netdevice reference leaks in attach_default_qdiscs()

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

net/sched: fix netdevice reference leaks in attach_default_qdiscs()

In attach_default_qdiscs(), if a dev has multiple queues and queue 0 fails
to attach qdisc because there is no memory in attach_one_default_qdisc().
Then dev->qdisc will be noop_qdisc by default. But the other queues may be
able to successfully attach to default qdisc.

In this case, the fallback to noqueue process will be triggered. If the
original attached qdisc is not released and a new one is directly
attached, this will cause netdevice reference leaks.

The following is the bug log:

veth0: default qdisc (fq_codel) fail, fallback to noqueue
unregister_netdevice: waiting for veth0 to become free. Usage count = 32
leaked reference.
qdisc_alloc+0x12e/0x210
qdisc_create_dflt+0x62/0x140
attach_one_default_qdisc.constprop.41+0x44/0x70
dev_activate+0x128/0x290
__dev_open+0x12a/0x190
__dev_change_flags+0x1a2/0x1f0
dev_change_flags+0x23/0x60
do_setlink+0x332/0x1150
__rtnl_newlink+0x52f/0x8e0
rtnl_newlink+0x43/0x70
rtnetlink_rcv_msg+0x140/0x3b0
netlink_rcv_skb+0x50/0x100
netlink_unicast+0x1bb/0x290
netlink_sendmsg+0x37c/0x4e0
sock_sendmsg+0x5f/0x70
____sys_sendmsg+0x208/0x280

Fix this bug by clearing any non-noop qdiscs that may have been assigned
before trying to re-attach.