8264572: ForkJoinPool.getCommonPoolParallelism() reports always 1

Reviewed-by: alanb

Author: Doug Lea

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

@@ -2572,6 +2572,7 @@ private ForkJoinPool(byte forCommonPoolOnly) {
         this.saturate = null;
         this.workerNamePrefix = null;
         int p = Math.min(Math.max(parallelism, 0), MAX_CAP), size;
+        this.mode = p;
         if (p > 0) {
             size = 1 << (33 - Integer.numberOfLeadingZeros(p - 1));
             this.bounds = ((1 - p) & SMASK) | (COMMON_MAX_SPARES << SWIDTH);