RFR: 8375981: G1: Convert G1RemSet helper classes to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review this conversion of `G1RemSet` helper classes to use `Atomic<>`.

There has been one additional non-volatile member that has been converted to use `Atomic<T>`.

Testing: gha, tier1

Thanks,
  Thomas

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

Commit messages:
 - 8375981

Changes: https://git.openjdk.org/jdk/pull/29345/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29345&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375981
  Stats: 28 lines in 1 file changed: 6 ins; 0 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/29345.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29345/head:pull/29345

PR: https://git.openjdk.org/jdk/pull/29345