RFR: 8313552: Fix -Wconversion warnings in JFR code

[Markus Grönlund]

Greetings,

this change set removes -Wconversion warnings in the JFR code.

Testing: jdk_jfr

Thanks
Markus

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Commit messages:
 - 8313552

Changes: https://git.openjdk.org/jdk/pull/15152/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15152&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313552
  Stats: 248 lines in 28 files changed: 21 ins; 11 del; 216 mod
  Patch: https://git.openjdk.org/jdk/pull/15152.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15152/head:pull/15152

PR: https://git.openjdk.org/jdk/pull/15152