thin_metadata_size: Fix long names of units

Both implementations of `thin_metadata_size` believe units are defined like this:

    1024 == kilobyte == k
    1000 == kibibyte == K

and so on.  Fix the 1000/1024 confusion, while continuing to follow the LVM convention of using lowercase letters for binary units, so that we have:

    1024 == kibibyte == k
    1000 == kilobyte == K