Bluetooth: Fix race condition in hci_cmd_sync_clear

There is a potential race condition in hci_cmd_sync_work and
hci_cmd_sync_clear, and could lead to use-after-free. For instance,
hci_cmd_sync_work is added to the 'req_workqueue' after cancel_work_sync
The entry of 'cmd_sync_work_list' may be freed in hci_cmd_sync_clear, and
causing kernel panic when it is used in 'hci_cmd_sync_work'.

Here's the call trace:

dump_stack_lvl+0x49/0x63
print_report.cold+0x5e/0x5d3
? hci_cmd_sync_work+0x282/0x320
kasan_report+0xaa/0x120
? hci_cmd_sync_work+0x282/0x320
__asan_report_load8_noabort+0x14/0x20
hci_cmd_sync_work+0x282/0x320
process_one_work+0x77b/0x11c0
? _raw_spin_lock_irq+0x8e/0xf0
worker_thread+0x544/0x1180
? poll_idle+0x1e0/0x1e0
kthread+0x285/0x320
? process_one_work+0x11c0/0x11c0
? kthread_complete_and_exit+0x30/0x30
ret_from_fork+0x22/0x30
</TASK>

Allocated by task 266:
kasan_save_stack+0x26/0x50
__kasan_kmalloc+0xae/0xe0
kmem_cache_alloc_trace+0x191/0x350
hci_cmd_sync_queue+0x97/0x2b0
hci_update_passive_scan+0x176/0x1d0
le_conn_complete_evt+0x1b5/0x1a00
hci_le_conn_complete_evt+0x234/0x340
hci_le_meta_evt+0x231/0x4e0
hci_event_packet+0x4c5/0xf00
hci_rx_work+0x37d/0x880
process_one_work+0x77b/0x11c0
worker_thread+0x544/0x1180
kthread+0x285/0x320
ret_from_fork+0x22/0x30

Freed by task 269:
kasan_save_stack+0x26/0x50
kasan_set_track+0x25/0x40
kasan_set_free_info+0x24/0x40
____kasan_slab_free+0x176/0x1c0
__kasan_slab_free+0x12/0x20
slab_free_freelist_hook+0x95/0x1a0
kfree+0xba/0x2f0
hci_cmd_sync_clear+0x14c/0x210
hci_unregister_dev+0xff/0x440
vhci_release+0x7b/0xf0
__fput+0x1f3/0x970
____fput+0xe/0x20
task_work_run+0xd4/0x160
do_exit+0x8b0/0x22a0
do_group_exit+0xba/0x2a0
get_signal+0x1e4a/0x25b0
arch_do_signal_or_restart+0x93/0x1f80
exit_to_user_mode_prepare+0xf5/0x1a0
syscall_exit_to_user_mode+0x26/0x50
ret_from_fork+0x15/0x30

Fixes: 6a98e383 ("Bluetooth: Add helper for serialized HCI command execution")
Cc: stable@vger.kernel.org
Signed-off-by: Min Li <lm0963hack@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>