RFR: 8319726: Parallel GC: Re-use object in object-iterator

[Roman Kennke]

As a minor follow-up/clean-up to [JDK-8319376](https://bugs.openjdk.org/browse/JDK-8319376), we can use the local obj field that we casted a few lines up, instead of casting from p again.

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Commit messages:
 - 8319726: Parallel GC: Re-use object in object-iterator

Changes: https://git.openjdk.org/jdk/pull/16565/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16565&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319726
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16565.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16565/head:pull/16565

PR: https://git.openjdk.org/jdk/pull/16565