RFR: 8277786: G1: Rename log2_card_region_per_heap_region used in G1CardSet

[Ivan Walulya]

Hi all,

Please review this small change to  rename log2_card_region_per_heap_region to log2_card_regions_per_heap_region in order to improve readability.

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Commit messages:
 - initial

Changes: https://git.openjdk.java.net/jdk/pull/6539/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6539&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277786
  Stats: 13 lines in 3 files changed: 0 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6539.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6539/head:pull/6539

PR: https://git.openjdk.java.net/jdk/pull/6539