RFR: 8256474: Migrate Mutex _owner accesses to use Atomic operations

[David Holmes]

Simple update to move away from volatile fields and use Atomic::load/store on racy accesses.

Thanks,
David

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Commit messages:
 - 8256474: Migrate Mutex _owner accesses to use Atomic operations

Changes: https://git.openjdk.java.net/jdk/pull/1402/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1402&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256474
  Stats: 25 lines in 2 files changed: 8 ins; 1 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1402.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1402/head:pull/1402

PR: https://git.openjdk.java.net/jdk/pull/1402