RFR: 8263464: NMT: assert in gtest os.release_multi_mappings_vm [v7]

[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.
> 
> The main idea here is to iterate over the "synthetic" region that was given to us and split it up into the underlaying existing regions, and then recursively handle them one at a time as we normally do.
> 
> 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:

  Johan's feedback: add comments, handle pesky NULL

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13813/files
  - new: https://git.openjdk.org/jdk/pull/13813/files/381bf693..01eeb5fd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13813&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13813&range=05-06

  Stats: 17 lines in 1 file changed: 16 ins; 0 del; 1 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