8351933: Inaccurate masking of TC subfield decrement in ForkJoinPool

Reviewed-by: phh, dl

Author: Dmitry Chuyko

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

@@ -1760,7 +1760,7 @@ private boolean tryTrim(WorkQueue w) {
             int sp = (int)c & ~INACTIVE;
             if ((sp & SMASK) == (cfg & SMASK) &&
                 compareAndSetCtl(c, ((pred & SP_MASK) |
-                                     (UC_MASK & (c - TC_UNIT))))) {
+                                     (c & RC_MASK) | ((c - TC_UNIT) & TC_MASK)))) {
                 w.config = cfg;  // add sentinel for deregisterWorker
                 w.phase = sp;
                 return true;