io_uring: use cond_resched() in sqthread

If preempt isn't enabled in the kernel, we can run into hang issues with
sqthread submissions. Use cond_resched() to play nice instead of
cpu_relax(), if we end up starting the loop and not having any events
pending for submissions.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2438,7 +2438,7 @@ static int io_sq_thread(void *data)
 			 * to sleep.
 			 */
 			if (inflight || !time_after(jiffies, timeout)) {
-				cpu_relax();
+				cond_resched();
 				continue;
 			}