bfs: split checkfs code into FileSystemVisitor and CheckVisitor

Another FileSystemVisitor implementation will be used for resizing

Change-Id: I2d5f47dcbefb4c76834487b194a99c6f5ae7de50
Reviewed-on: https://review.haiku-os.org/c/923
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>