RFR: 8323021: Shenandoah: Encountered reference count always attributed to first worker thread

[William Kemper]

Simple fix, code was using array base pointer instead of indexing by worker id as intended.

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Commit messages:
 - Increment encountered count for correct worker

Changes: https://git.openjdk.org/jdk/pull/17273/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17273&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323021
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17273.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17273/head:pull/17273

PR: https://git.openjdk.org/jdk/pull/17273