RFR: 8314114: Fix -Wconversion warnings in os code, primarily linux [v2]

[Coleen Phillimore]

On Mon, 14 Aug 2023 02:03:22 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> The reason I changed it was to assign into size_t below without sign conversion.
>
> Sure, but can't it be size_t itself in the first place?

Ok, doesn't reintroduce warnings to change back to size_t.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15229#discussion_r1293959632