Fix synchronization in ByteSizeCachingDirectory (elastic#52512)

One particular code place was synchronizing on the wrong object.

server/src/main/java/org/elasticsearch/index/store/ByteSizeCachingDirectory.java

@@ -160,7 +160,7 @@ public void close() throws IOException {
                 try {
                     super.close();
                 } finally {
-                    synchronized (this) {
+                    synchronized (ByteSizeCachingDirectory.this) {
                         numOpenOutputs--;
                         modCount++;
                     }