xtables-addons: Fix Lua packet script implementation

lua_packet_segment parameter start has type char pointer; in function lua_tg
it's assigned an uint16 value generating compiler warnings obviously indicating
posssible seg fault problems. Fix the issue by using the correct skb functions
so the parameter points to the position inside the sk_buff

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
Signed-off-by: Stijn Cleynhens <stijn.cleynhens@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49331 3c298f89-4303-0410-b956-a3cf2f4a3e73