RFR: 8335930: GenShen: Reserve regions within each generation's freeset until available is sufficient

[Kelvin Nilsen]

Hi all,
This pull request contains a backport of commit 472e4c74 from the openjdk/shenandoah repository.
The commit being backported was authored by Kelvin Nilsen on 11 Jul 2024 and was reviewed by William Kemper.
Thanks!

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Commit messages:
 - Backport 472e4c74bde1c0ceb76755165e709cefa4fef584

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/73/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=73&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335930
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/73.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/73/head:pull/73

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/73