RFR: 8312127: FileDescriptor.sync should temporarily increase parallelism

[Alan Bateman]

If FileDescriptor::sync is invoked by a virtual thread then it doesn't release its carrier to do other work while modifications are written to the file system. For cases like this, parallelism should be temporarily increased for the duration of the operation. This one is somewhat legacy as it's only JDK 1.0/1.1 era APIs that provide access to the FileDescriptor so it wasn't done with the other methods in JDK 19. It's updated here, as it has been noticed.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/14893/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14893&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312127
  Stats: 17 lines in 3 files changed: 11 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/14893.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14893/head:pull/14893

PR: https://git.openjdk.org/jdk/pull/14893