RFR: 8298061: vmTestbase/nsk/sysdict/vm/stress/btree/btree012/btree012.java failed with "fatal error: refcount has gone to zero"

[Coleen Phillimore]

Please review this limited fix to make symbol refcount not go to zero for the placeholder supername field.  This supername field is to detect ClassCircularityError for parallel capable class loaders without deadlocking.
ie. t1 : Class A (lock object for A) -> super B (wait for lock object for B)
     t2: Class B (lock object for B) -> super A (wait for lock object for A).

We get the supername field out of the placeholder table to do the check for super class CCE but the reference count  in the placeholder table for this name can be one.  The fix is not to decrement and increment if the name is the same.

Tested with tier1-4.

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Commit messages:
 - 8298061: vmTestbase/nsk/sysdict/vm/stress/btree/btree012/btree012.java failed with "fatal error: refcount has gone to zero"

Changes: https://git.openjdk.org/jdk/pull/11726/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11726&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298061
  Stats: 12 lines in 2 files changed: 3 ins; 3 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/11726.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11726/head:pull/11726

PR: https://git.openjdk.org/jdk/pull/11726