RFR: 8267953: restore 'volatile' to ObjectMonitor::_owner field

[Daniel D.Daugherty]

A trivial fix to restore 'volatile' to ObjectMonitor::_owner field.

This fix was tested with a Mach5 Tier job set.

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Commit messages:
 - 8267953: restore 'volatile' to ObjectMonitor::_owner field

Changes: https://git.openjdk.java.net/jdk/pull/4262/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4262&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267953
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4262.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4262/head:pull/4262

PR: https://git.openjdk.java.net/jdk/pull/4262