HDFS-16003. ProcessReport print invalidatedBlocks should judge debug level at first. Contributed by lei w.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlockManager.java

@@ -2794,9 +2794,12 @@ public boolean processReport(final DatanodeID nodeID,
       namesystem.writeUnlock();
     }
 
-    for (Block b : invalidatedBlocks) {
-      blockLog.debug("BLOCK* processReport 0x{}: {} on node {} size {} does not"
-          + " belong to any file", strBlockReportId, b, node, b.getNumBytes());
+    if(blockLog.isDebugEnabled()) {
+      for (Block b : invalidatedBlocks) {
+        blockLog.debug("BLOCK* processReport 0x{}: {} on node {} size {} " +
+                        "does not belong to any file.", strBlockReportId, b,
+                         node, b.getNumBytes());
+      }
     }
 
     // Log the block report processing stats from Namenode perspective