Parsing GRE packets to get inner IPv4 original source and destr addresses

Hi,

I was trying to find an example of GRE packet inspection but couldn't find one. I'm trying to get the original IPv4 source and destination addresses.

```
int incoming(struct __sk_buff *skb)
{
void *data = (void *)(long)skb->data;
void *data_end = (void *)(long)skb->data_end;

struct iphdr *inner_ip; <--- Need this

return TC_ACT_OK;
}
```

Any ideas?