Modify taskqueue_drain_all() processing to use a temporary "barrier task", rather than rely on a user task that may be destroyed during taskqueue_drain_all()'s execution. The barrier task is queued behind all previously queued tasks and then has its priority elevated so that future tasks cannot pass it in the queue. Use a similar barrier scheme to drain threads processing current tasks. This requires taskqueue_run_locked() to insert and remove the taskqueue_busy object for the running thread for every task processed.
Remove warning about live-lock issues with taskqueue_drain_all().