RFR: 8313564: Fix -Wconversion warnings in classfile code [v2]

[Coleen Phillimore]

On Wed, 2 Aug 2023 23:31:17 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> I think each value needs to be cast to get the correct result.
>
> Yes size_t values must be cast to float/double. Again I think double should be used here.

Also changed to double.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/15111#discussion_r1282517582