RFR: 8306738: Select num workers for safepoint ParallelCleanupTask [v2]

[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:

  Use active_workers

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13616/files
  - new: https://git.openjdk.org/jdk/pull/13616/files/390ca675..1acfd501

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13616&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13616&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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