Ignore detected block sizes in cdrom provider

[mraerino]

- What I did

Ignored detected blocksizes in go-diskfs. We were running into an issue where the detection is wrong (https://github.com/diskfs/go-diskfs/issues/104) which causes a panic in metadata (https://github.com/diskfs/go-diskfs/issues/103)

- How I did it

- How to verify it

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[deitch]

This makes sense to me. There are open PRs on go-diskfs to fix this specific issue (the panics), but I see no reason not to rely on default sizes here.