blk-iolatency: error out if blk_get_queue() failed in iolatency_set_limit()

If queue is dying while iolatency_set_limit() is in progress,
blk_get_queue() won't increment the refcount of the queue. However,
blk_put_queue() will still decrement the refcount later, which will
cause the refcout to be unbalanced.

Thus error out in such case to fix the problem.

Fixes: 8c772a9b ("blk-iolatency: fix IO hang due to negative inflight counter")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20210805124645.543797-1-yukuai3@huawei.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iolatency.c

@@ -833,7 +833,11 @@ static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 
 	enable = iolatency_set_min_lat_nsec(blkg, lat_val);
 	if (enable) {
-		WARN_ON_ONCE(!blk_get_queue(blkg->q));
+		if (!blk_get_queue(blkg->q)) {
+			ret = -ENODEV;
+			goto out;
+		}
+
 		blkg_get(blkg);
 	}