8351933: Inaccurate masking of TC subfield decrement in ForkJoinPool

Reviewed-by: dl, alanb, liach

Author: Dmitry Chuyko

src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java

@@ -2077,7 +2077,8 @@ private int tryTrim(WorkQueue w, int phase, long deadline) {
         else if (deadline - System.currentTimeMillis() >= TIMEOUT_SLOP)
             stat = 0;                       // spurious wakeup
         else if (!compareAndSetCtl(
-                     c, nc = (w.stackPred & LMASK) | (UMASK & (c - TC_UNIT))))
+                     c, nc = ((w.stackPred & LMASK) | (RC_MASK & c) |
+                               (TC_MASK & (c - TC_UNIT)))))
             stat = -1;                      // lost race to signaller
         else {
             stat = 1;