[jdk21u] RFR: 8312127: FileDescriptor.sync should temporarily increase parallelism

[Aleksey Shipilev]

Clean backport to avoid the temporary loss of platform thread when virtual thread does `fsync`. In https://github.com/openjdk/jdk/pull/15231, I tested that the overhead of doing this for platform threads is very small compared to the cost of the `fsync` itself.

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Commit messages:
 - Backport fbe51e388d8a7475f162f10aa788a6d37ec4c6d8

Changes: https://git.openjdk.org/jdk21u/pull/52/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=52&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/jdk21u/pull/52.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/52/head:pull/52

PR: https://git.openjdk.org/jdk21u/pull/52