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

[Thomas Schatzl]

On Fri, 18 Sep 2020 06:59:34 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> 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.

Re-approve (if needed) after latest discussion.

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Marked as reviewed by tschatzl (Reviewer).

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