RFR: 8301641: NativeMemoryUsageTotal event uses reserved value for committed field

[Stefan Johansson]

Please review this small fix for the NativeMemoryUsageTotal JFR event.

**Summary**
The NativeMemoryUsageTotal event uses the reserved value for both reserved and committed.

**Testing**
Added new test and verified that it works locally. Now running it in mach5 to make sure it doesn't see any unexpected issues there.

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Commit messages:
 - Update test comment
 - Add test
 - 8301641: NativeMemoryUsageTotal event uses reserved value for committed field

Changes: https://git.openjdk.org/jdk/pull/12377/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12377&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301641
  Stats: 13 lines in 2 files changed: 12 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12377.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12377/head:pull/12377

PR: https://git.openjdk.org/jdk/pull/12377