Prefix index implementation for utf8 string is incorrect

[birdstorm]

Related to pingcap/tidb#7104

mysql> show create table t1;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                         |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `name` varchar(12) DEFAULT NULL,
  KEY `pname` (`name`(12))
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> insert into t1 values('借款策略集_网页');
Query OK, 1 row affected (0.14 sec)

scala> spark.sql("select * from t1 where name = '借款策略集_网页'").show
+----+
|name|
+----+
+----+

scala> spark.sql("select * from t1 where name < '借款策略集_网页'").show
+----+
|name|
+----+
|借款策略|
+----+