RFR: 8361056: Parallel: Use correct is_par argument in ScavengeRootsTask

[Albert Mingkun Yang]

Trivial fixing the hardcoded `is_par` arg used during young-gc.

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Commit messages:
 - pgc-trivial

Changes: https://git.openjdk.org/jdk/pull/26039/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26039&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361056
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26039.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26039/head:pull/26039

PR: https://git.openjdk.org/jdk/pull/26039