io_uring: cancel work if task_work_add() fails

We currently move it to the io_wqe_manager for execution, but we cannot
safely do so as we may lack some of the state to execute it out of
context. As we cancel work anyway when the ring/task exits, just mark
this request as canceled and io_async_task_func() will do the right
thing.

Fixes: aa96bf8a ("io_uring: use io-wq manager as backup task if task is exiting")
Signed-off-by: Jens Axboe <axboe@kernel.dk>