fix push to middle bug

x-pack/plugin/blob-cache/src/main/java/org/elasticsearch/blobcache/shared/SharedBlobCacheService.java

@@ -1319,7 +1319,7 @@ private void pushEntryToMiddle(final LRUCacheEntry entry) {
                 middle.head = entry;
                 middle.tail = entry;
             } else {
-                entry.next = middle.head.next;
+                entry.next = middle.head;
                 middle.head.prev = entry;
                 middle.head = entry;
             }