[jdk21u-dev] RFR: 8320525: G1: G1UpdateRemSetTrackingBeforeRebuild::distribute_marked_bytes accesses partially unloaded klass

[Goetz Lindenmaier]

I backport this as prereq of [JDK-8317809](https://bugs.openjdk.org/browse/JDK-8317809).

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Commit messages:
 - Backport 21d361e944a599e82e5ed93aa81ad485b4601843

Changes: https://git.openjdk.org/jdk21u-dev/pull/517/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=517&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320525
  Stats: 10 lines in 1 file changed: 0 ins; 7 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/517.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/517/head:pull/517

PR: https://git.openjdk.org/jdk21u-dev/pull/517