RFR: 8242032: G1 region remembered sets may contain non-coarse level PRTs for already coarsened regions

[Joakim Nordström]

This fix adds a check for coarsened region in mutex guarded section, when adding a reference to a remembered set.

Haven't been able to produce a testcase -- please advice on how to, or if not necessary.

**Testing:**
* hs-tier, hs-tier2

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Commit messages:
 - Added check for coarsened region in mutex guarded section.

Changes: https://git.openjdk.java.net/jdk/pull/2545/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2545&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8242032
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2545.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2545/head:pull/2545

PR: https://git.openjdk.java.net/jdk/pull/2545