RFR: 8373627: assert(!is_vthread_transition_disabler()) failed: no suspend allowed for vthread transition disablers

[Serguei Spitsyn]

This is a 1-liner fix of a regression.
The issue was discovered after merge with main line of this PR: https://github.com/openjdk/jdk/pull/28407 .

It will need to be back ported to 26 as well.

Testing:
 - TBD: This was already tested in another PR repo but will submit mach5 tiers 1-3 to be completely safe.

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Commit messages:
 - 8373627: assert(!is_vthread_transition_disabler()) failed: no suspend allowed for vthread transition disablers

Changes: https://git.openjdk.org/jdk/pull/28805/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28805&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373627
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28805.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28805/head:pull/28805

PR: https://git.openjdk.org/jdk/pull/28805