RFR: 8280375: G1: Tighten mem region limit in G1RebuildRemSetHeapRegionClosure

[Hamlin Li]

Currently, it uses `hr->end()` to guard the mem scan limit, but it should be top instead of end.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/7154/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7154&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280375
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7154.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7154/head:pull/7154

PR: https://git.openjdk.java.net/jdk/pull/7154