ChaosData and yonghong-song examples/http_filter: Add pointer offset accounting for variable ip h…
…eader len (#1868)

This fix adds additional accounting logic to the http_filter examples ("simple"
and "complete") that make sure to shift the pointer for the IP body/payload to
the correct offset before accessing TCP header fields. This is done by taking
into account the IP header length field. Previously, the IP header length field
was used, but it was done later in processing, after TCP header values were
extracted using the size of the BCC `proto.h` `struct ip_t` a static offset.

Prior to this, it was possible to evade HTTP detection by injecting IP options
data into the IP header that would spoof parts of the TCP header and shift the
real one down, as done in the scapy snippet below:

Latest commit 3d9b687 Jul 19, 2018