RFR: 8242032: G1 region remembered sets may contain non-coarse level PRTs for already coarsened regions [v2]

[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

Joakim Nordström has updated the pull request incrementally with two additional commits since the last revision:

 - Clarified comment on re-checking coarsening
 - Minor typo

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2545/files
  - new: https://git.openjdk.java.net/jdk/pull/2545/files/7398c89b..4bc0f4b7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2545&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2545&range=00-01

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