RFR: 8275381 G1: refactor 2 constructors of G1CardSetConfiguration

[Hamlin Li]

2 constructors of G1CardSetConfiguration share some code, it's better to refacoring them to simplify the code.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6033/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6033&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275381
  Stats: 62 lines in 2 files changed: 27 ins; 21 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6033.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6033/head:pull/6033

PR: https://git.openjdk.java.net/jdk/pull/6033