mac802154: fix use-after-free of sdata via queued RX frames

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

mac802154: fix use-after-free of sdata via queued RX frames

The RX softirq producer ieee802154_subif_frame() queues received beacon
and MAC-command frames onto local->rx_beacon_list / rx_mac_cmd_list and
schedules a process-context worker, storing a raw mac_pkt->sdata (and
skb->dev == sdata->dev) with neither a reference nor any locking:

- the lists have no lock: the softirq producer list_add_tail()s while the
mac_wq worker list_del()s, so sibling interfaces on the same phy corrupt
the list;

- the workers dereference the interface after it may have been freed.
mac802154_rx_mac_cmd_worker() touches mac_pkt->sdata directly, and
mac802154_rx_beacon_worker() -> mac802154_process_beacon() dereferences
skb->dev (== sdata->dev). Removing an interface frees its sdata
(netdev_priv) while a queued frame still points at it, so a later worker
run is a use-after-free.

Reproduced under KASAN by flooding a victim interface with MAC command
frames and removing it (the beacon path is the same class via skb->dev):

BUG: KASAN: slab-use-after-free in mac802154_rx_mac_cmd_worker+0x463/0x630 [mac802154]
Read of size 4 at addr ffff888002f9ea18 by task kworker/u8:1/31
Workqueue: phy0-mac-cmds mac802154_rx_mac_cmd_worker [mac802154]
Call Trace:
mac802154_rx_mac_cmd_worker+0x463/0x630 [mac802154]
process_one_work+0x611/0xe80
worker_thread+0x52e/0xdc0
kthread+0x30c/0x630
ret_from_fork+0x2fd/0x3e0

Fix both lists together:

- add local->rx_lock and take it around every list access: the softirq
producer (plain spin_lock, softirq context) and the workers and flush
(spin_lock_bh, process context);

- pin the interface for the lifetime of a queued frame with
netdev_hold()/netdev_put(), so the worker can safely dereference sdata /
skb->dev even while the interface is being removed;

- dequeue under the lock at the head and loop-drain the whole list in the
workers (they previously processed one frame per run and relied on a
later enqueue to drain the rest);

- drop not-yet-started frames of an interface before it is unregistered,
from ieee802154_if_remove() (after the RCU grace period) and from the
ieee802154_remove_interfaces() loop -- the latter is the whole-phy
teardown path, which does not go through ieee802154_if_remove().

An in-flight worker that already dequeued a frame keeps its own netdev
reference; unregister_netdevice() then waits it out in netdev_run_todo(),
which runs at rtnl_unlock() (rtnl released) and after the interface has
been closed, so it does not pin rtnl. A worker blocked in an association
TX only delays that one interface's unregister (the usual "waiting for %s
to become free"), it does not hold rtnl. netdev_hold() is used for this
reason instead of a cancel_work_sync() under rtnl, which would block on
the worker's unbounded MLME TX wait via ieee802154_sync_queue().

The mac-command worker additionally skips processing for a stopped
interface (ieee802154_sdata_running()), avoiding a needless association
response during teardown.