[HDFS-15789] Lease renewal does not require namesystem lock. Contributed by Daryn Sharp and Jim Brennan.

(cherry picked from commit f565476939332c2b17a0f470ac650ab55c6c0f64)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java

@@ -4019,14 +4019,10 @@ public class FSNamesystem implements Namesystem, FSNamesystemMBean,
    */
   void renewLease(String holder) throws IOException {
     checkOperation(OperationCategory.WRITE);
-    readLock();
-    try {
-      checkOperation(OperationCategory.WRITE);
-      checkNameNodeSafeMode("Cannot renew lease for " + holder);
-      leaseManager.renewLease(holder);
-    } finally {
-      readUnlock("renewLease");
-    }
+    checkNameNodeSafeMode("Cannot renew lease for " + holder);
+    // fsn is not mutated so lock is not required.  the leaseManger is also
+    // thread-safe.
+    leaseManager.renewLease(holder);
   }
 
   /**