RFR: 8350567: NMT: update VMATree::register_mapping to copy the existing tag of the region

[Johan Sjölen]

On Tue, 25 Feb 2025 10:01:17 GMT, Afshin Zafari <azafari at openjdk.org> wrote:

> When committing a sub-region (SR) in the middle of a reserved region (RR), we need to decide on the MemTag. To find the correct tag, we had to find the RR base and take the tag and use it for SR.
> With this PR, there will be no need to find the RR base and the tag of the previous region of SR can be copied to the SR.
> 
> Tests:
> linux-x64-debug, gtest:NMT*, runtime/NMT

I thought that this was what `use_tag_inplace` did from the start? There's some difference in behavior here that I do not understand. I'd like to see a test introduced which does not pass before, but passes now.

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PR Review: https://git.openjdk.org/jdk/pull/23771#pullrequestreview-2644321105