RFR: 8342730: Get rid of SummaryDiff in VMATree [v2]

[Afshin Zafari]

> `SummaryDiff` result of any `register_mapping()` call, is passed into the function rather than returned. This enables avoiding copying the structure to/from the stack.
> 
> Tests:
> local linux-x64-debug: NMT gtests and JTREGs passed.

Afshin Zafari has updated the pull request incrementally with one additional commit since the last revision:

  better naming for diff

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26761/files
  - new: https://git.openjdk.org/jdk/pull/26761/files/e1c88ff2..9b53507d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26761&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26761&range=00-01

  Stats: 89 lines in 2 files changed: 0 ins; 0 del; 89 mod
  Patch: https://git.openjdk.org/jdk/pull/26761.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26761/head:pull/26761

PR: https://git.openjdk.org/jdk/pull/26761