RFR: 8361948: Shenandoah: region free capacity unit mismatch

[William Kemper]

Clean backport. Fixes Shenandoah unit mismatch bug.

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Commit messages:
 - Backport 46988e1073e9a2b47491c90143b1f261fe56da56

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/213/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=213&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361948
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/213.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/213/head:pull/213

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/213