RFR 8228490: Shenandoah: Shenandoah concurrent root evacuation may race against OopStorage::release()

[Roman Kennke]

We have the fix in shenandoah/jdk already, and it seems to resolve the
trouble. Patch looks good!

Thanks,
Roman

> Concurrent evacuating OopStorage backed roots, may race against
> OopStorage::release() calls. Therefore, it needs to update the roots
> with Atomic::cmpxchg(), to avoid overwriting the roots if they are
> released.
> 
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8228490
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8228490/webrev.00/index.html
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release)
>   jcstress fastdebug
>   nightly tests
> 
> Thanks,
> 
> -Zhengyu