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

[Afshin Zafari]

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

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Commit messages:
 - 8350567: NMT: update VMATree::register_mapping to copy the existing tag of the region

Changes: https://git.openjdk.org/jdk/pull/23771/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23771&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350567
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23771.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23771/head:pull/23771

PR: https://git.openjdk.org/jdk/pull/23771