RFR: JDK-8302455: VM.classloader_stats memory size values are wrong

[Thomas Stuefe]

VM.classloader_stats shows metaspace consumption in words, but should show bytes. 

Patch fixes that, and adjusts the associated jtreg test to catch regressions like this.

I manually tested the test with the unpatched hotspot to see if the regression shows up, it does.

Tested the patch on x64 (fastdebug, release) and x86.

Note: "calculate_jfr_stats" is misnamed, actually only ever used by this dcmd, not by jfr. I will clear this up in a separate RFE.

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Commit messages:
 - Fix VM.classloader_stats

Changes: https://git.openjdk.org/jdk/pull/12556/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12556&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302455
  Stats: 19 lines in 2 files changed: 15 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/12556.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12556/head:pull/12556

PR: https://git.openjdk.org/jdk/pull/12556