[jdk21u-dev] RFR: 8322255: Generational ZGC: ZPageSizeMedium should be set before MaxTenuringThreshold

[Liang Mao]

Clean backport for fixing generational ZGC. The commit fixes the incorrect calculation of MaxTenuringThreshold and the risk is quite low.

Additional testing:
 - [x] Linux aarch64 server fastdebug, `hotspot_gc` with `-XX:+UseZGC +XX+ZGenerational`

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Commit messages:
 - Backport 0ad6c9e3d91005c0cc3a26d5391444c3dcd8ba5d

Changes: https://git.openjdk.org/jdk21u-dev/pull/210/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=210&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322255
  Stats: 5 lines in 2 files changed: 3 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/210.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/210/head:pull/210

PR: https://git.openjdk.org/jdk21u-dev/pull/210