btrfs: comment the rest of implicit barriers before waitqueue_active

There are atomic operations that imply the barrier for waitqueue_active
mixed in an if-condition.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -802,6 +802,9 @@ static void run_one_async_done(struct btrfs_work *work)
 	limit = btrfs_async_submit_limit(fs_info);
 	limit = limit * 2 / 3;
 
+	/*
+	 * atomic_dec_return implies a barrier for waitqueue_active
+	 */
 	if (atomic_dec_return(&fs_info->nr_async_submits) < limit &&
 	    waitqueue_active(&fs_info->async_submit_wait))
 		wake_up(&fs_info->async_submit_wait);