[jdk11u-dev] RFR: JDK-8215889 assert(!_unloading) failed: This oop is not available to unloading class loader data with ZGC

[Peter Zhelezniakov]

Almost clean backport. I've just changed the type of `PrepareExtraDataClosure::_safepoint_counter` from `uint64_t` to `int` to match the type of `SafepointSynchronize::_safepoint_counter` and thus avoid a compiler warning.

This fix had a flaw that was later identified and corrected under JDK-8231501, so this latter fix will also be backported.

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

Commit messages:
 - JDK-8215889 assert(!_unloading) failed: This oop is not available to unloading class loader data with ZGC

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/19/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=19&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8215889
  Stats: 117 lines in 8 files changed: 103 ins; 10 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/19.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/19/head:pull/19

PR: https://git.openjdk.java.net/jdk11u-dev/pull/19