[improve][broker] Avoid PersistentReplicator.expireMessages logic compute backlog twice (#23957)

Author: hanmz

pulsar-broker/src/main/java/org/apache/pulsar/broker/service/persistent/PersistentReplicator.java

@@ -670,8 +670,8 @@ public void updateMessageTTL(int messageTTLInSeconds) {
 
     @Override
     public boolean expireMessages(int messageTTLInSeconds) {
-        if ((cursor.getNumberOfEntriesInBacklog(false) == 0)
-                || (cursor.getNumberOfEntriesInBacklog(false) < MINIMUM_BACKLOG_FOR_EXPIRY_CHECK
+        long backlog = cursor.getNumberOfEntriesInBacklog(false);
+        if ((backlog == 0) || (backlog < MINIMUM_BACKLOG_FOR_EXPIRY_CHECK
                         && !topic.isOldestMessageExpired(cursor, messageTTLInSeconds))) {
             // don't do anything for almost caught-up connected subscriptions
             return false;