MDEV-16779 Assertion !rw_lock_own failed upon purge

This is a regression caused by the fix of MDEV-15855.

purge_vcol_info_t::set_used(): Add a missing condition.

row_purge_poss_sec(): Invoke set_used() in order to
have !is_first_fetch() when retrying.

storage/innobase/include/row0types.h

@@ -92,7 +92,9 @@ struct purge_vcol_info_t
 			return;
 		}
 
-		first_use = used = true;
+		if (!used) {
+			first_use = used = true;
+		}
 	}
 
 	/** Check whether it fetches mariadb table for the first time.

storage/innobase/row/row0purge.cc

@@ -345,6 +345,7 @@ row_purge_poss_sec(
 
 	if (node->vcol_info.is_first_fetch()) {
 		if (node->vcol_info.mariadb_table) {
+			node->vcol_info.set_used();
 			goto retry_purge_sec;
 		}