Add level 255 support

[saindriches]

We don't know much about it, and only guarantee that the generated data is accepted by verifier.

[quietvoid]

Not sure why the verifier would accept it. Documentation says:

These metadata levels, if present, should be located in dm_data_payload(): 0, 1, 2, 4, 5, 6, and 255.

So the 255 should be along DM v1 blocks (or CM v2.9 in the code).

And when trying to verify with your code:

Invalid DM metadata level location [E238]. Found level 255 extension in payload 2

[saindriches]

You're right, I was focused on if the verifier parses blocks correctly (using -rt) and was unaware of this part. It should be a dmv1 block, will fix it.

[quietvoid]

Looks fine and works on my side. I'll add the C API part later.

Thanks!