lua/bpf: support for NET_OFF for dissector

the packet can now use indirect addressing mode
using NET_OFF to read from network protocol off

it has convenience dissectors for IPv4 and IPv6,
that can be used like:

```
local net = pkt.net_off
if net.ver == 4 then
    local ip = net.ip
else
    local ip6 = net.ip6
end
```

src/lua/bpf/cdef.lua

@@ -78,6 +78,9 @@ struct bpf {
 	static const int F_USER_STACK     = 1 << 8;
 	static const int F_FAST_STACK_CMP = 1 << 9;
 	static const int F_REUSE_STACKID  = 1 << 10;
+	/* special offsets for ancillary data */
+	static const int NET_OFF          = -0x100000;
+	static const int LL_OFF           = -0x200000;
 };
 /* eBPF commands */
 struct bpf_cmd {

src/lua/bpf/proto.lua

@@ -35,6 +35,10 @@ struct sk_buff {
 	uint32_t tc_classid;
 };
 
+struct net_off_t {
+	uint8_t  ver:4;
+} __attribute__((packed));
+
 struct eth_t {
 	uint8_t  dst[6];
 	uint8_t  src[6];
@@ -275,12 +279,23 @@ M.udp     = function (...) return dissector(ffi.typeof('struct udp_t'), ...) end
 M.tcp     = function (...) return dissector(ffi.typeof('struct tcp_t'), ...) end
 M.vxlan   = function (...) return dissector(ffi.typeof('struct vxlan_t'), ...) end
 M.data    = function (...) return dissector(ffi.typeof('uint8_t'), ...) end
+M.net_off = function (...) return dissector(ffi.typeof('struct net_off_t'), ...) end
 
 -- Metatables
 ffi.metatype(ffi.typeof('struct eth_t'), {
 	__index = {
 		ip = skip_eth,
 		ip6 = skip_eth,
+		net_off = function (e, dst)
+			next_skip(e, dst, BPF.NET_OFF)
+		end,
+	}
+})
+
+ffi.metatype(ffi.typeof('struct net_off_t'), {
+	__index = {
+		ip = function () end,
+		ip6 = function () end,
 	}
 })