aio: Skip timer for io_getevents if timeout=0

In this case, it is basically a polling. Let's not involve timer at all
because that would hurt performance for application event loops.

In an arbitrary test I've done, io_getevents syscall elapsed time
reduces from 50000+ nanoseconds to a few hundereds.
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>

fs/aio.c

@@ -1253,8 +1253,12 @@ static long read_events(struct kioctx *ctx, long min_nr, long nr,
 	 * the ringbuffer empty. So in practice we should be ok, but it's
 	 * something to be aware of when touching this code.
 	 */
-	wait_event_interruptible_hrtimeout(ctx->wait,
-			aio_read_events(ctx, min_nr, nr, event, &ret), until);
+	if (until.tv64 == 0)
+		aio_read_events(ctx, min_nr, nr, event, &ret);
+	else
+		wait_event_interruptible_hrtimeout(ctx->wait,
+				aio_read_events(ctx, min_nr, nr, event, &ret),
+				until);
 
 	if (!ret && signal_pending(current))
 		ret = -EINTR;