RFR: 8345543: Test serviceability/jvmti/vthread/StopThreadTest/StopThreadTest.java failed: expected JVMTI_ERROR_OPAQUE_FRAME instead of: 0

[Serguei Spitsyn]

The problem is that the state of the target `vthread` is changed to `BLOCKED` too early. It is done in a constructor of helper object `JavaThreadBlockedOnMonitorEnterState` which sets the state of `vthread` to `JavaThreadStatus::BLOCKED_ON_MONITOR_ENTER`. The `vthread` is getting suspended in `try_preempt()` while calling `JvmtiVTMSTransitionDisabler::start_VTMS_transition()`, so does not become unmounted as it is expected by the failing test. Also, it seems that the base class of the `JavaThreadBlockedOnMonitorEnterState` is supporting just HotSpot  `JavaThread`'s and so, it is changing the carrier thread state only.

The fix is to move the definition of the `JavaThreadBlockedOnMonitorEnterState` object after block where `try_preempt()` is called.

Testing:
 - Checked the failed test does not fail with the fix: `serviceability/jvmti/vthread/StopThreadTest/StopThreadTest.java`
 - Ran mach5 tiers 1-6

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Commit messages:
 - 8345543: Test serviceability/jvmti/vthread/StopThreadTest/StopThreadTest.java failed: expected JVMTI_ERROR_OPAQUE_FRAME instead of: 0

Changes: https://git.openjdk.org/jdk/pull/23125/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23125&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345543
  Stats: 6 lines in 1 file changed: 3 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23125.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23125/head:pull/23125

PR: https://git.openjdk.org/jdk/pull/23125