writeback: fix bdi_writeback iteration in wakeup_dirtytime_writeback()

wakeup_dirtytime_writeback() walks and wakes up all wb's of all bdi's;
unfortunately, it was always waking up bdi->wb instead of the wb being
walked.  Fix it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 001fe6f6 ("writeback: make wakeup_dirtytime_writeback() handle multiple bdi_writeback's")
Reviewed-by: Jan Kara <jack@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

fs/fs-writeback.c

@@ -1897,8 +1897,8 @@ static void wakeup_dirtytime_writeback(struct work_struct *w)
 		struct wb_iter iter;
 
 		bdi_for_each_wb(wb, bdi, &iter, 0)
-			if (!list_empty(&bdi->wb.b_dirty_time))
-				wb_wakeup(&bdi->wb);
+			if (!list_empty(&wb->b_dirty_time))
+				wb_wakeup(wb);
 	}
 	rcu_read_unlock();
 	schedule_delayed_work(&dirtytime_work, dirtytime_expire_interval * HZ);