RFR: 8365994: ZGC: Incorrect type signature in ZMappedCache comparator

[Joel Sikström]

Hello,

The comparator with two IntrusiveRBNode* in ZMappedCache has the incorrent type signature. This prevents IntrusiveRBTree from using the comparator as intended during validation, resulting in it using the fallback verify function, which always returns true. This could mask potential issues in the tree structure. 

Testing:
* Manually placing an assert in the fallback verify function to see if it is used. It is no longer used with this patch.
* Oracle's tier 1-2

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Commit messages:
 - 8365994: ZGC: Incorrect type signature in ZMappedCache comparator

Changes: https://git.openjdk.org/jdk/pull/26904/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26904&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365994
  Stats: 8 lines in 2 files changed: 0 ins; 3 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/26904.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26904/head:pull/26904

PR: https://git.openjdk.org/jdk/pull/26904