MDEV-18512 using DATETIME(6) as row_start/row_end crashes server

Disallow DATETIME for SYSTEM VERSIONING tables.

mysql-test/suite/versioning/r/create.result

@@ -508,5 +508,12 @@ row_end bigint as row end,
 period for system_time (row_start, row_end)
 ) engine=myisam with system versioning;
 ERROR HY000: `row_start` must be of type TIMESTAMP(6) for system-versioned table `t1`
+create table t (
+a int,
+row_start datetime(6) generated always as row start,
+row_end datetime(6) generated always as row end,
+period for system_time(row_start, row_end)
+) with system versioning;
+ERROR HY000: `row_start` must be of type TIMESTAMP(6) for system-versioned table `t`
 drop database test;
 create database test;

mysql-test/suite/versioning/t/create.test

@@ -387,5 +387,14 @@ create or replace table t1 (
   period for system_time (row_start, row_end)
 ) engine=myisam with system versioning;
 
+--error ER_VERS_FIELD_WRONG_TYPE
+create table t (
+  a int,
+  row_start datetime(6) generated always as row start,
+  row_end datetime(6) generated always as row end,
+  period for system_time(row_start, row_end)
+) with system versioning;
+
+
 drop database test;
 create database test;

sql/handler.cc

@@ -7433,8 +7433,7 @@ bool Vers_parse_info::check_conditions(const Lex_table_name &table_name,
 
 static bool is_versioning_timestamp(const Create_field *f)
 {
-  return (f->type_handler() == &type_handler_datetime2 ||
-          f->type_handler() == &type_handler_timestamp2) &&
+  return f->type_handler() == &type_handler_timestamp2 &&
          f->length == MAX_DATETIME_FULL_WIDTH;
 }
 

sql/table.cc

@@ -2025,7 +2025,6 @@ int TABLE_SHARE::init_from_binary_frm_image(THD *thd, bool write,
         switch (field_type)
         {
         case MYSQL_TYPE_TIMESTAMP2:
-        case MYSQL_TYPE_DATETIME2:
           break;
         case MYSQL_TYPE_LONGLONG:
           if (vers_can_native)