8365557: Parallel: Refactor ParallelScavengeHeap::mem_allocate_work

[albertnetymk]

Extract out multi-thread-safe old-gen allocation API call outside the critical-region to streamline the flow in mem_allocate_work.

Test: tier1-3

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• JDK-8365557: Parallel: Refactor ParallelScavengeHeap::mem_allocate_work (Enhancement - P4)

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