buf_flush_batch(): Remove the test page_count != ULINT_UNDEFINED.

The variable is initialized to zero, and after that it is only added to.
Maybe the one who introduced the variable srv_buf_pool_flushed overlooked
that there is a separate return statement for returning ULINT_UNDEFINED?

buf/buf0flu.c

@@ -977,8 +977,7 @@ buf_flush_batch(
 	}
 #endif /* UNIV_DEBUG */
 
-	if (page_count != ULINT_UNDEFINED)
-		srv_buf_pool_flushed+= page_count;
+	srv_buf_pool_flushed += page_count;
 
 	return(page_count);
 }