RFR: 8350567: NMT: update VMATree::register_mapping to copy the existing tag of the region [v2]

[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

Afshin Zafari has updated the pull request incrementally with two additional commits since the last revision:

 - removed extra whitespace.
 - unit test added.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23771/files
  - new: https://git.openjdk.org/jdk/pull/23771/files/5d85bb8c..50a75b22

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23771&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23771&range=00-01

  Stats: 16 lines in 1 file changed: 16 ins; 0 del; 0 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