Bluetooth: MGMT: free the mesh send cancel command when it is cancelled

In the Linux kernel, the following vulnerability has been resolved: Bluetooth: MGMT: free the mesh send cancel command when it is cancelled mesh_send_cancel() queues the pending command with a NULL destroy callback, so it is only freed if send_cancel() runs. A cancelled entry is leaked, as _hci_cmd_sync_cancel_entry() does not release entry->data when there is no destroy callback, and hci_cmd_sync_clear() cancels every pending entry when the controller is unregistered. Nothing else reclaims it either: mgmt_pending_new() does not put the command on hdev->mgmt_pending. The leak also pins the socket reference taken by mgmt_pending_new(), so the mgmt socket is never released. Free the command from a destroy callback.