[jdk25] RFR: 8360201: JFR: Initialize JfrThreadLocal::_sampling_critical_section

[Aleksey Shipilev]

Fixes the bug in the new feature in JDK 25. The fix is pretty trivial.

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Commit messages:
 - Backport 5c1f77fab1f56e470157d8680310417924abb298

Changes: https://git.openjdk.org/jdk/pull/25952/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25952&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360201
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25952.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25952/head:pull/25952

PR: https://git.openjdk.org/jdk/pull/25952