Handle forward traverse in roll back operation.

[piste-jp]

What I'm saying here is that ltfsck can list rollback points via forward traversal

I decided this behavior intentionally before. But I can't remind why it is not a good behavior at all. Another behavior change might make it's not a problem anymore..

Originally posted by @piste-jp-ibm in https://github.com/LinearTapeFileSystem/ltfs/pull/302#discussion_r689187545