RFR: 8324970: Serial: Refactor signature of maintain_old_to_young_invariant

[Albert Mingkun Yang]

Trivial type-change in signature.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17631/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17631&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324970
  Stats: 6 lines in 2 files changed: 1 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17631.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17631/head:pull/17631

PR: https://git.openjdk.org/jdk/pull/17631