8387387: Parallel: Clean up startup allocation locking

[albertnetymk]

Simple cleanup in ParallelScavengeHeap::mem_allocate_work, which restructures the pre-initialization heap expansion check to use ConditionalMutexLocker for InitCompleted_lock, matching the nearby Heap_lock pattern, and polishes the comments around the locking invariant. There is no intended behavior change.

Test: tier1

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Issue

• JDK-8387387: Parallel: Clean up startup allocation locking (Enhancement - P4)

Reviewers

• Thomas Schatzl (@tschatzl - Reviewer)

Contributors

• Axel Boldt-Christmas <aboldtch@openjdk.org>

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[albertnetymk]

/contributor add aboldtch

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