xtables: another try at chain name length checking

Since XT_EXTENSION_MAXNAMELEN is now available, make use of it
and clear the confusion.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/iptables.c b/iptables.c
index 19c2af5..efe993e 100644
--- a/iptables.c
+++ b/iptables.c
@@ -460,10 +460,10 @@
 		xtables_error(PARAMETER_PROBLEM,
 			   "Invalid target name (too short)");
 
-	if (strlen(targetname) > XT_FUNCTION_MAXNAMELEN - 1)
+	if (strlen(targetname) >= XT_EXTENSION_MAXNAMELEN)
 		xtables_error(PARAMETER_PROBLEM,
 			   "Invalid target name `%s' (%u chars max)",
-			   targetname, XT_FUNCTION_MAXNAMELEN - 1);
+			   targetname, XT_EXTENSION_MAXNAMELEN - 1);
 
 	for (ptr = targetname; *ptr; ptr++)
 		if (isspace(*ptr))