RFR: 8291725: Leftover marks when VM shutdown aborts bitmap clearing make mixed gc fail

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that removes stray bits on the mark bitmap during mixed gcs during shutdown?

So it may happen that after shutdown aborts the "Concurrent Cleanup for Next Mark" phase which leaves marks on the bitmaps (we want to shut down asap after all). However a mixed gc afterwards might notice these marks and in this case asserts, but in case of an evacuation failure might also trip over these additional marks.

Testing: tier1-5, I did not manage to reproduce this, but it's "obvious" given the failure mode, particularly because recently there has been a similar issue with full gc in [JDK-8274007](https://bugs.openjdk.org/browse/JDK-8274007)

Thanks,
  Thomas

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Commit messages:
 - Initial version

Changes: https://git.openjdk.org/jdk/pull/9735/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9735&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291725
  Stats: 23 lines in 1 file changed: 16 ins; 6 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9735.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9735/head:pull/9735

PR: https://git.openjdk.org/jdk/pull/9735