RFR: bad oop with JvmtiVTMSTransitionDisabler

[Alex Menkov]

Bad oop can happen after JvmtiVTMSTransitionDisabler is installed (at java_lang_VirtualThread::carrier_thread(java_thread) call).
Fixed by converting java_thread to Handle

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Commit messages:
 - obsolete comment
 - bad oop after JvmtiVTMSTransitionDisabler

Changes: https://git.openjdk.org/loom/pull/217/files
  Webrev: https://webrevs.openjdk.org/?repo=loom&pr=217&range=00
  Stats: 11 lines in 1 file changed: 2 ins; 4 del; 5 mod
  Patch: https://git.openjdk.org/loom/pull/217.diff
  Fetch: git fetch https://git.openjdk.org/loom.git pull/217/head:pull/217

PR: https://git.openjdk.org/loom/pull/217