RFR: 8296492: Remove ObjectLocker in JVMTI get_subgroups call

[Serguei Spitsyn]

On Tue, 8 Nov 2022 00:58:44 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> The JVM code took a ThreadGroup lock before poking into ThreadGroup fields.  Call a method in the ThreadGroup to call the synchronized method instead.
> Tested with tier 1-4.

This looks nice in general.
But I will make another pass.

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PR: https://git.openjdk.org/jdk/pull/11033