RFR: 8329898: Revert one use of markWord.is_unlocked back to is_neutral

[David Holmes]

[JDK-8325303](https://bugs.openjdk.org/browse/JDK-8325303) changed a number of occurrences of `is_neutral` to `is_unlocked`, but one change in basicLock.cpp should not have been changed as it relates to the displaced markword.

This is a trivial change.

Testing:
 - tiers 1-3 (sanity)

Thanks

-------------

Commit messages:
 - 8329898: Revert one use of markWord.is_unlocked back to is_neutral

Changes: https://git.openjdk.org/jdk/pull/18684/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18684&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329898
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18684.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18684/head:pull/18684

PR: https://git.openjdk.org/jdk/pull/18684