RFR: 8311556: GetThreadLocalStorage not working for vthreads mounted during JVMTI attach [v2]

[Serguei Spitsyn]

> This is an issue with a dynamic load of a JVMTI agent into running VM.
> The `VM_SetNotifyJvmtiEventsMode` enabling operation makes a call to the function `count_transitions_and_correct_jvmti_thread_states()`. This function in its turn make a call to the function `correct_jvmti_thread_state()`. But it does it conditionally, only if the field `_whitebox_used` is `true`.
> The test provided in the bug report showed that it has to be called unconditionally as the assumption that it is only needed on the subsequent `notifyJvmti` enabling is incorrect.
> 
> Then the field `_whitebox_used` is not needed anymore and removed in this fix.
> Some obsolete comments are removed or updated.
> 
> New test is added: `test/hotspot/jtreg/serviceability/jvmti/vthread/VThreadTLSTest`.
> It is failed without the fix and passed with the fix. 
> 
> Testing:
>   - ran new test `test/hotspot/jtreg/serviceability/jvmti/vthread/VThreadTLSTest`
>   - mach5 tiers 1-5 are good

Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:

  review: removed unneeded @compile commands from new test

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/14842/files
  - new: https://git.openjdk.org/jdk/pull/14842/files/f582fbc0..743188b5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14842&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14842&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14842.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14842/head:pull/14842

PR: https://git.openjdk.org/jdk/pull/14842