io_uring: transform ret == 0 for poll cancelation completions

We can set canceled == true and complete out-of-line, ensure that we catch
that and correctly return -ECANCELED if the poll operation got canceled.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4955,6 +4955,9 @@ static void io_poll_complete(struct io_kiocb *req, __poll_t mask, int error)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 
+	if (!error && req->poll.canceled)
+		error = -ECANCELED;
+
 	io_poll_remove_double(req);
 	req->poll.done = true;
 	io_cqring_fill_event(req, error ? error : mangle_poll(mask));