Use-after-free of an L2CAP CoC channel object in the Zephyr Bluetooth host: RX work item is not cancelled on channel teardown

The Bluetooth LE host queues received L2CAP connection-oriented channel (CoC) data for deferred processing through a struct k_work embedded in the channel object (le_chan->rx_work, handler l2cap_rx_process()) whenever the channel uses a dynamic PSM (0x0080-0x00FF). Channel teardown in l2cap_chan_destroy() in subsys/bluetooth/host/l2cap.c cancels the retransmission-timeout work and drains the RX FIFO, but never cancels rx_work. Because that work item was submitted to the system workqueue while HCI receive processing runs on the dedicated Bluetooth RX workqueue (CONFIG_BT_RECV_WORKQ_BT, the default), a queued rx_work item can outlive the channel it points into. A remote, unauthenticated peer with an established CoC channel triggers this by sending a data K-frame immediately followed by an L2CAP Disconnect Request. The K-frame submits rx_work to the system workqueue; because both workqueue threads are cooperative and the Bluetooth RX workqueue runs at the higher priority (K_PRIO_COOP(CONFIG_BT_RX_PRIO) versus CONFIG_SYSTEM_WORKQUEUE_PRIORITY), the pending item cannot run before the following Disconnect Request is processed in le_disconn_req() -> l2cap_chan_del() -> l2cap_chan_destroy(). The stack then invokes the released() callback, which the API documents as meaning the stack has dropped all references and the application may free the channel memory. The application therefore frees or re-accepts into an object that the system workqueue still holds in its pending list. If the m