RFR: 8263464: NMT: assert in gtest os.release_multi_mappings_vm [v2]

[Gerard Ziemski]

> This fix allows NMT to account for released memory that was allocated in chunks, as long as the final pointer and size refer to total contiguous regions of the requested size.
> 
> Tested via `Mach5 tier1,tier2,tier3` and locally via `gtest:NMT*:os*`

Gerard Ziemski has updated the pull request incrementally with one additional commit since the last revision:

  support partial release, add corresponding test

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13813/files
  - new: https://git.openjdk.org/jdk/pull/13813/files/c6ad4daa..7d6a6c0c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13813&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13813&range=00-01

  Stats: 52 lines in 2 files changed: 46 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13813.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13813/head:pull/13813

PR: https://git.openjdk.org/jdk/pull/13813