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

[Daniel D.Daugherty]

On Sat, 29 May 2021 17:29:56 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> A trivial fix to restore 'volatile' to ObjectMonitor::_owner field.
>> 
>> This fix was tested with a Mach5 Tier job set.
>
> Yes. I very much approve.

@theRealAph - Thanks for the review!

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