RFR: 8274196: Crashes in VM_HeapDumper::work after JDK-8252842

[Lin Zang]

The root cause for crash in ZGC is that the JNIHandles are processed before object iteration. And ZGC would update the JNIHandles at object iteration with read barrier. So the crash is cause by accessing the invalid address which can be dummy info after zgc, and hence crash.

The lock rank issue can be fixed because the related mutexes are acquired in safepoint. so the safepoint_check_required could be safepoint_check_always.

The Epsilon issue is caused by wrong _num_dumper_thread calculated when the gang==NULL.

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Commit messages:
 - 8274196: Crashes in VM_HeapDumper::work after JDK-8252842

Changes: https://git.openjdk.java.net/jdk/pull/5681/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5681&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274196
  Stats: 48 lines in 2 files changed: 22 ins; 23 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5681.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5681/head:pull/5681

PR: https://git.openjdk.java.net/jdk/pull/5681