RFR: 8306738: Select num workers for safepoint ParallelCleanupTask [v3]

[Axel Boldt-Christmas]

> Select num workers for safepoint ParallelCleanupTask to be at most the number of parallel cleanup tasks `SAFEPOINT_CLEANUP_NUM_TASKS`
> 
> Unsure if `WorkerThreads::max_workers()` is more correct than `WorkerThreads::created_workers()` or `WorkerThreads::active_workers()`. 
> 
> Tested tier1-3 Oracle platforms.

Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:

  Add expected_num_workers

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13616/files
  - new: https://git.openjdk.org/jdk/pull/13616/files/1acfd501..b319d608

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13616&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13616&range=01-02

  Stats: 61 lines in 7 files changed: 49 ins; 4 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/13616.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13616/head:pull/13616

PR: https://git.openjdk.org/jdk/pull/13616