fw3: zones: limit zone names to 11 bytes

As defined in currently used iptables v1.8.4, chain name must be
under 29 chars. Thus, user can only edit 11 chars.

See also the bugreport [0].

[0] https://dev.archive.openwrt.org/ticket/20380

Signed-off-by: Alexey Dobrovolsky <dobrovolskiy.alexey@gmail.com>
[improve commit description, move length calculation information to src]
Signed-off-by: David Bauer <mail@david-bauer.net>

zones.h

@@ -22,8 +22,12 @@
 #include "options.h"
 #include "iptables.h"
 
-/* 32 - sizeof("postrouting_") - sizeof("_rule") - sizeof("\0") */
-#define FW3_ZONE_MAXNAMELEN 14
+/* XT_EXTENSION_MAXNAMELEN (29)
+ *  - sizeof("postrouting_")
+ *  - sizeof("_rule")
+ *  - sizeof("\0")
+ */
+#define FW3_ZONE_MAXNAMELEN 11
 
 extern const struct fw3_option fw3_zone_opts[];