blk-wbt: call rq_qos_add() after wb_normal is initialized

Our test found a problem that wbt inflight counter is negative, which
will cause io hang(noted that this problem doesn't exist in mainline):

t1: device create	t2: issue io
add_disk
 blk_register_queue
  wbt_enable_default
   wbt_init
    rq_qos_add
    // wb_normal is still 0
			/*
			 * in mainline, disk can't be opened before
			 * bdev_add(), however, in old kernels, disk
			 * can be opened before blk_register_queue().
			 */
			blkdev_issue_flush
                        // disk size is 0, however, it's not checked
                         submit_bio_wait
                          submit_bio
                           blk_mq_submit_bio
                            rq_qos_throttle
                             wbt_wait
			      bio_to_wbt_flags
                               rwb_enabled
			       // wb_normal is 0, inflight is not increased

    wbt_queue_depth_changed(&rwb->rqos);
     wbt_update_limits
     // wb_normal is initialized
                            rq_qos_track
                             wbt_track
                              rq->wbt_flags |= bio_to_wbt_flags(rwb, bio);
			      // wb_normal is not 0，wbt_flags will be set
t3: io completion
blk_mq_free_request
 rq_qos_done
  wbt_done
   wbt_is_tracked
   // return true
   __wbt_done
    wbt_rqw_done
     atomic_dec_return(&rqw->inflight);
     // inflight is decreased

commit 8235b5c1e8 ("block: call bdev_add later in device_add_disk") can
avoid this problem, however it's better to fix this problem in wbt:

1) Lower kernel can't backport this patch due to lots of refactor.
2) Root cause is that wbt call rq_qos_add() before wb_normal is
initialized.

Fixes: e34cbd3074 ("blk-wbt: add general throttling mechanism")
Cc: <stable@vger.kernel.org>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20220913105749.3086243-1-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-wbt.c

@@ -843,6 +843,10 @@ int wbt_init(struct request_queue *q)
 	rwb->enable_state = WBT_STATE_ON_DEFAULT;
 	rwb->wc = 1;
 	rwb->rq_depth.default_depth = RWB_DEF_DEPTH;
+	rwb->min_lat_nsec = wbt_default_latency_nsec(q);
+
+	wbt_queue_depth_changed(&rwb->rqos);
+	wbt_set_write_cache(q, test_bit(QUEUE_FLAG_WC, &q->queue_flags));
 
 	/*
 	 * Assign rwb and add the stats callback.
@@ -853,11 +857,6 @@ int wbt_init(struct request_queue *q)
 
 	blk_stat_add_callback(q, rwb->cb);
 
-	rwb->min_lat_nsec = wbt_default_latency_nsec(q);
-
-	wbt_queue_depth_changed(&rwb->rqos);
-	wbt_set_write_cache(q, test_bit(QUEUE_FLAG_WC, &q->queue_flags));
-
 	return 0;
 
 err_free: