luci-app-firewall: limit zone name length to 11 characters

Change the maximum length of a firewall zone name from 14 to 11 characters.

Longer names break iptables rule generation (max. 29 chars are allowed).

XT_EXTENSION_MAXNAMELEN = 29
29 - sizeof("postrouting_") - sizeof("_rule") - sizeof("\0") = 11

References to:
https://github.com/openwrt/luci/issues/507
https://dev.openwrt.org/ticket/20380

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>