bfs: add stub for ResizeVisitor.

This allows to have all the tools (resizefs, fs_shell, etc) merged, and
split out the remaining BFS resize changes in a way that makes some
sense. We can easily merg

bfs: add stub for ResizeVisitor.

This allows to have all the tools (resizefs, fs_shell, etc) merged, and
split out the remaining BFS resize changes in a way that makes some
sense. We can easily merge all the basic infrastructure (ioctls, etc)
without any of the actual resizing code (I'll leave that to
professionals).

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