Bluetooth: hci_sync: add lock to protect HCI_UNREGISTER

Bluetooth: hci_sync: add lock to protect HCI_UNREGISTER

[ Upstream commit 1857c19941c87eb36ad47f22a406be5dfe5eff9f ]

When the HCI_UNREGISTER flag is set, no jobs should be scheduled. Fix
potential race when HCI_UNREGISTER is set after the flag is tested in
hci_cmd_sync_queue.

Fixes: 0b94f2651f56 ("Bluetooth: hci_sync: Fix queuing commands when HCI_UNREGISTER is set")
Signed-off-by: Zhengping Jiang <jiangzp@google.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>