MDEV-21563 FTS thread aborts during shutdown

Problem:
=======
After discarding the table, fts_optimize_thread aborts during shutdown.
InnoDB fails to remove the table from fts_optimize_wq and it leads to
the fts_optimize_thread to lookup for the auxiliary table and fails.

Fix:
====
While discarding the fts table, remove the table from fts_optimize_wq.

storage/innobase/row/row0mysql.cc

@@ -3205,6 +3205,16 @@ row_discard_tablespace_for_mysql(
 		err = DB_ERROR;
 
 	} else {
+		bool fts_exist = (dict_table_has_fts_index(table)
+				  || DICT_TF2_FLAG_IS_SET(
+					  table, DICT_TF2_FTS_HAS_DOC_ID));
+
+		if (fts_exist) {
+			row_mysql_unlock_data_dictionary(trx);
+			fts_optimize_remove_table(table);
+			row_mysql_lock_data_dictionary(trx);
+		}
+
 		/* Do foreign key constraint checks. */
 
 		err = row_discard_tablespace_foreign_key_checks(trx, table);
@@ -3212,6 +3222,10 @@ row_discard_tablespace_for_mysql(
 		if (err == DB_SUCCESS) {
 			err = row_discard_tablespace(trx, table);
 		}
+
+		if (fts_exist && err != DB_SUCCESS) {
+			fts_optimize_add_table(table);
+		}
 	}
 
 	return(row_discard_tablespace_end(trx, table, err));

storage/xtradb/row/row0mysql.cc

@@ -3209,6 +3209,16 @@ row_discard_tablespace_for_mysql(
 		err = DB_ERROR;
 
 	} else {
+		bool fts_exist = (dict_table_has_fts_index(table)
+				  || DICT_TF2_FLAG_IS_SET(
+					  table, DICT_TF2_FTS_HAS_DOC_ID));
+
+		if (fts_exist) {
+			row_mysql_unlock_data_dictionary(trx);
+			fts_optimize_remove_table(table);
+			row_mysql_lock_data_dictionary(trx);
+		}
+
 		/* Do foreign key constraint checks. */
 
 		err = row_discard_tablespace_foreign_key_checks(trx, table);
@@ -3216,6 +3226,11 @@ row_discard_tablespace_for_mysql(
 		if (err == DB_SUCCESS) {
 			err = row_discard_tablespace(trx, table);
 		}
+
+		if (fts_exist && err != DB_SUCCESS) {
+			fts_optimize_add_table(table);
+		}
+
 	}
 
 	return(row_discard_tablespace_end(trx, table, err));