RFR: 8324048: (fc) Make FileKey fields final [v2]

[Brian Burkhalter]

On Wed, 21 Aug 2024 19:18:28 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

>> I think the important part here is that libnio defines `Java_sun_nio_ch_FileKey_init`.
>
> Something must be loading the library but I don't know what. I did some testing which does not use any NIO classes other than `FileKey`, and the class worked without the explicit load, on both Unix and Windows. I think, however, that I need to add it on Unix and restore it on Windows to be safe.

Added / reinstated `IOUtil.load()` in 378d700.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20429#discussion_r1725654386