RFR: 8294278: ForkJoinPool.getAndAddPoolIds should use Unsafe.staticFieldBase

[Alan Bateman]

Two (since 19) usages of Unsafe.getAndAddInt to update a static field provide the Class object as the base instead of the base object returned by Unsafe.staticFieldBase. This doesn't change anything on the HotSpot VM.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/11369/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11369&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294278
  Stats: 17 lines in 2 files changed: 12 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/11369.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11369/head:pull/11369

PR: https://git.openjdk.org/jdk/pull/11369