Let xtradb set mrr_length_per_rec stats.

storage/xtradb/handler/ha_innodb.cc

@@ -7639,6 +7639,7 @@ ha_innobase::info(
 		}
 
 		stats.check_time = 0;
+		stats.mrr_length_per_rec= ref_length +  8; // 8 = max(sizeof(void *));
 
 		if (stats.records == 0) {
 			stats.mean_rec_length = 0;