MDEV-25910: Make ALTER TABLE...ALGORITHM=COPY durable again

ha_innobase::extra(HA_EXTRA_END_ALTER_COPY): Flush the redo log
because the main transaction will not have written any log and
thus will not initiate any log write either. But the DDL recovery
layer expects that the operation will have been committed.

storage/innobase/handler/ha_innodb.cc

@@ -15308,6 +15308,7 @@ ha_innobase::extra(
 		break;
 	case HA_EXTRA_END_ALTER_COPY:
 		m_prebuilt->table->skip_alter_undo = 0;
+		log_write_up_to(LSN_MAX, true);
 		break;
 	default:/* Do nothing */
 		;