blk-iolatency: clear use_delay when io.latency is set to zero

If use_delay was non-zero when the latency target of a cgroup was set
to zero, it will stay stuck until io.latency is enabled on the cgroup
again.  This keeps readahead disabled for the cgroup impacting
performance negatively.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Josef Bacik <jbacik@fb.com>
Fixes: d706751215 ("block: introduce blk-iolatency io controller")
Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iolatency.c

@@ -778,8 +778,10 @@ static int iolatency_set_min_lat_nsec(struct blkcg_gq *blkg, u64 val)
 
 	if (!oldval && val)
 		return 1;
-	if (oldval && !val)
+	if (oldval && !val) {
+		blkcg_clear_delay(blkg);
 		return -1;
+	}
 	return 0;
 }