8185623: (se) AbstractSelector.selectorOpen is not published safely

Make 'selectorOpen' instance variable final

Reviewed-by: chegar
diff --git a/jdk/src/java.base/share/classes/java/nio/channels/spi/AbstractSelector.java b/jdk/src/java.base/share/classes/java/nio/channels/spi/AbstractSelector.java
index 3c4eda9..5d710bd 100644
--- a/jdk/src/java.base/share/classes/java/nio/channels/spi/AbstractSelector.java
+++ b/jdk/src/java.base/share/classes/java/nio/channels/spi/AbstractSelector.java
@@ -70,7 +70,7 @@
     extends Selector
 {
 
-    private AtomicBoolean selectorOpen = new AtomicBoolean(true);
+    private final AtomicBoolean selectorOpen = new AtomicBoolean(true);
 
     // The provider that created this selector
     private final SelectorProvider provider;