RFR: 8276663: Cleanup NMT AccessLock [v3]

[Zhengyu Gu]

> Please review this cleanup patch to NMT AccessLock:
> 
> * Make LockState enum private, no external use.
> * const lock location pointer
> * Use memory_order_relaxed for lock counter updates.
> 
> Test:
> - [x] hotspot_nmt on x86_64 and aarch64

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Fix memory ordering

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6267/files
  - new: https://git.openjdk.java.net/jdk/pull/6267/files/d3cb57b6..6b430244

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6267&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6267&range=01-02

  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6267.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6267/head:pull/6267

PR: https://git.openjdk.java.net/jdk/pull/6267