MDEV-26626 InnoDB fails to advance the log checkpoint

buf_flush_page_cleaner(): Always try to advance the log checkpoint,
even when no pages were flushed during the latest batch.
Maybe, since the previous batch, there was an LRU flush that
removed the last dirty pages.

Failure to advance the log checkpoint will cause unnecessary work
in Mariabackup and on crash recovery.

storage/innobase/buf/buf0flu.cc

@@ -2266,6 +2266,15 @@ static os_thread_ret_t DECLARE_THREAD(buf_flush_page_cleaner)(void*)
 unemployed:
       buf_flush_async_lsn= 0;
       buf_pool.page_cleaner_set_idle(true);
+
+      DBUG_EXECUTE_IF("ib_log_checkpoint_avoid", continue;);
+
+      mysql_mutex_unlock(&buf_pool.flush_list_mutex);
+
+      if (!recv_recovery_is_on() && srv_operation == SRV_OPERATION_NORMAL)
+        log_checkpoint();
+
+      mysql_mutex_lock(&buf_pool.flush_list_mutex);
       continue;
     }