HDFS-16798. HDFS-16798. SerialNumberMap should decrease current counter if the item exist. (#4987). Contributed by ZanderXu.

Signed-off-by: He Xiaoqiao <hexiaoqiao@apache.org>

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

@@ -68,6 +68,7 @@ public class SerialNumberMap<T> {
       }
       Integer old = t2i.putIfAbsent(t, sn);
       if (old != null) {
+        current.getAndDecrement();
         return old;
       }
       i2t.put(sn, t);