os-helpers-fs: move logic to assert partition structure to helper file

[alexgg]

Move the generic dmcrypt partition assertion logic. This is slightly different as paritions do not contain meta-data that can identify the encrypted partitions so dmsetup is used after the partition is mounted.

Change-type: patch

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[ycardaillac]

Hi @alexgg, I'm wondering why what was a one liner, became this more complex function?
It's seems that this function measures the size that a file is going to take in zram by compressing the file manually and counting the blocks.
Was there a bug with the du command?
Why is it always zram compressed now? vs never compressed (du -cb)?
Is it that we were we were actually requiring the raw image size, when we actually needed the compressed one?
If not what was the problem and how was it solved?
I haven't yet really gotten into HUP and updating the OS and the associated complexity sorry if I'm lacking a bit of context.