MDEV-21282 Assertion 'mariadb_table' failed in gcol.innodb_virtual_debug_purge

- commit ea37b144 (MDEV-16678) caused
a regression. when purge thread tries to open the table for virtual
column computation, there is no need to acquire MDL for the table.
Because purge thread already hold MDL for the table

mysql-test/suite/gcol/r/gcol_purge.result

+CREATE TABLE t1(f1 INT NOT NULL, f2 int not null,
+f3 int generated always as (f2 * 2) VIRTUAL,
+primary key(f1), INDEX (f3))ENGINE=InnoDB;
+connect con1,localhost,root,,,;
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+connection default;
+INSERT INTO t1(f1, f2) VALUES(1,2);
+DELETE from t1 where f1 = 1;
+connect con2,localhost,root,,,;
+begin;
+INSERT INTO t1 (f1, f2) VALUES(1,2);
+set global debug_dbug="+d,ib_purge_virtual_index_callback";
+connection con1;
+COMMIT;
+InnoDB		0 transactions not purged
+connection con2;
+commit;
+disconnect con1;
+disconnect con2;
+connection default;
+set global debug_dbug=default;
+DROP TABLE t1;

mysql-test/suite/gcol/t/gcol_purge.test

+--source include/have_innodb.inc
+--source include/have_debug.inc
+CREATE TABLE t1(f1 INT NOT NULL, f2 int not null,
+		f3 int generated always as (f2 * 2) VIRTUAL,
+		primary key(f1), INDEX (f3))ENGINE=InnoDB;
+connect(con1,localhost,root,,,);
+START TRANSACTION WITH CONSISTENT SNAPSHOT;
+
+connection default;
+INSERT INTO t1(f1, f2) VALUES(1,2);
+DELETE from t1 where f1 = 1;
+
+connect(con2,localhost,root,,,);
+begin;
+INSERT INTO t1 (f1, f2) VALUES(1,2);
+
+set global debug_dbug="+d,ib_purge_virtual_index_callback";
+connection con1;
+COMMIT;
+
+--source ../innodb/include/wait_all_purged.inc
+
+connection con2;
+commit;
+
+disconnect con1;
+disconnect con2;
+connection default;
+set global debug_dbug=default;
+DROP TABLE t1;

sql/sql_class.cc

@@ -4716,7 +4716,8 @@ TABLE *open_purge_table(THD *thd, const char *db, size_t dblen,
   DBUG_ASSERT(thd->open_tables == NULL);
   DBUG_ASSERT(thd->locked_tables_mode < LTM_PRELOCKED);
 
-  Open_table_context ot_ctx(thd, 0);
+  /* Purge already hold the MDL for the table */
+  Open_table_context ot_ctx(thd, MYSQL_OPEN_HAS_MDL_LOCK);
   TABLE_LIST *tl= (TABLE_LIST*)thd->alloc(sizeof(TABLE_LIST));
   LEX_CSTRING db_name= {db, dblen };
   LEX_CSTRING table_name= { tb, tblen };