RFR: 8341554: Shenandoah: Missing heap lock when updating usage for soft ref policy

[William Kemper]

A recent change to avoid checking the log level under the lock inadvertently removed the lock from an operation that needs it.

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Commit messages:
 - Restore missing heap lock when updating usage at last gc

Changes: https://git.openjdk.org/jdk/pull/21362/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21362&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341554
  Stats: 7 lines in 1 file changed: 3 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21362.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21362/head:pull/21362

PR: https://git.openjdk.org/jdk/pull/21362