RFR: 8253286: Use expand_exact() instaed of expand_at() for fixed requests in G1

[Stefan Johansson]

Hi all,

Please review this small cleanup that uses `expand_exact()` rather than `expand_at()` when requesting a specific
region. This will ensure that we get exactly the requested region.

Testing:
tier1-2 alone and 3-5 with other changes.

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Commit messages:
 - 8253286: Use expand_exact() instaed of expand_at() for fixed requests in G1

Changes: https://git.openjdk.java.net/jdk/pull/235/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=235&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253286
  Stats: 7 lines in 1 file changed: 1 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/235.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/235/head:pull/235

PR: https://git.openjdk.java.net/jdk/pull/235