RFR: 8276047: G1: refactor G1CardSetArrayLocker

[Albert Mingkun Yang]

Simple refactoring in `G1CardSetArrayLocker`:

1. rename fields to sth more explicit
2. drop `volatile` for thread-local variables
3. increment directly without treating `bool` as an integer

Test: hotspot_gc

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

Commit messages:
 - cleanup

Changes: https://git.openjdk.java.net/jdk/pull/6134/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6134&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276047
  Stats: 21 lines in 2 files changed: 3 ins; 4 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6134.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6134/head:pull/6134

PR: https://git.openjdk.java.net/jdk/pull/6134