RFR: 8366658: Add missing locks when accessing the VirtualMemoryTracker instance in tests and MemMapPrinter

[Johan Sjölen]

Hi,

This PR adds the missing locks for the tests and MemMapPrinter, also fixes a small bug in `walk_virtual_memory`. See inline review comments.

Passes tier1.

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Commit messages:
 - Missing locks
 - Fix
 - Fix
 - Add missing locks

Changes: https://git.openjdk.org/jdk/pull/27038/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27038&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366658
  Stats: 53 lines in 8 files changed: 27 ins; 1 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/27038.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27038/head:pull/27038

PR: https://git.openjdk.org/jdk/pull/27038