RFR: 8334769: Shenandoah: Move CodeCache_lock close to its use in ShenandoahConcurrentNMethodIterator

[Zhengyu Gu]

ShenandoahConcurrentNMethodIterator requires CodeCache_lock at begin and end of iteration.

Currently, the CodeCache_lock is acquired by their callers, which is not very obvious and readable. Let's move the lock inside the methods.

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Commit messages:
 - 8334769: Shenandoah: Move CodeCache_lock close to its use in ShenandoahConcurrentNMethodIterator

Changes: https://git.openjdk.org/jdk/pull/19859/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19859&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334769
  Stats: 13 lines in 3 files changed: 3 ins; 8 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19859.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19859/head:pull/19859

PR: https://git.openjdk.org/jdk/pull/19859