RFR: 8286704: G1: Call offset_of directly in subclasses of G1CardSetContainer

[Albert Mingkun Yang]

Simple cleanup in subclasses of `G1CardSetContainer`.

Test: build

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Commit messages:
 - offset-of

Changes: https://git.openjdk.java.net/jdk/pull/8698/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8698&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286704
  Stats: 33 lines in 2 files changed: 12 ins; 18 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8698.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8698/head:pull/8698

PR: https://git.openjdk.java.net/jdk/pull/8698