RFR: 8366363: MemBaseline accesses VMT without using lock

[Johan Sjölen]

Hi,

The `MemBaseline` used to access the VMT instance directly without a lock. We fix that, and we switch from using a `LinkedList` to a copied `RegionsTree` instead.

-------------

Commit messages:
 - Remove now unused iterator
 - Use a copied VMATree instead of a LinkedList

Changes: https://git.openjdk.org/jdk/pull/27003/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27003&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366363
  Stats: 140 lines in 10 files changed: 75 ins; 44 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/27003.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27003/head:pull/27003

PR: https://git.openjdk.org/jdk/pull/27003