[jdk21u] RFR: 8316967: Correct the scope of vmtimer in UnregisteredClasses::load_class

[Aleksey Shipilev]

Clean backport to improve perf counter reliability.

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Commit messages:
 - Backport 83b6a1278f7ffa5bf6f30697e5fb39eb5846c04b

Changes: https://git.openjdk.org/jdk21u/pull/393/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=393&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316967
  Stats: 9 lines in 3 files changed: 0 ins; 6 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u/pull/393.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/393/head:pull/393

PR: https://git.openjdk.org/jdk21u/pull/393