addendum - minor optimization

Author: virajjasani

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/impl/prefetch/SingleFilePerBlockCache.java

@@ -322,6 +322,10 @@ private void addToHeadOfLinkedList(Entry entry) {
     if (entry != head) {
       Entry prev = entry.getPrevious();
       Entry nxt = entry.getNext();
+      // no-op if the block is already evicted
+      if (!blocks.containsKey(entry.blockNumber)) {
+        return;
+      }
       if (prev != null) {
         prev.setNext(nxt);
       }