blk-iolatency: deal with small samples

There is logic to keep cgroups that haven't done a lot of IO in the most
recent scale window from being punished for over-active higher priority
groups.  However for things like ssd's where the windows are pretty
short we'll end up with small numbers of samples, so 5% of samples will
come out to 0 if there aren't enough.  Make the floor 1 sample to keep
us from improperly bailing out of scaling down.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iolatency.c

@@ -366,7 +366,7 @@ static void check_scale_change(struct iolatency_grp *iolat)
 		 * scale down event.
 		 */
 		samples_thresh = lat_info->nr_samples * 5;
-		samples_thresh = div64_u64(samples_thresh, 100);
+		samples_thresh = max(1ULL, div64_u64(samples_thresh, 100));
 		if (iolat->nr_samples <= samples_thresh)
 			return;
 	}