There is no need for appdata pointer: net_pkt's cursor is already at the
right position, i.e. the beginning of the payload right after all IP/TCP
headers.
Also, when reading the actual data, let's use net_pkt_read_new()
relevantly instead of going through the buffer by ourselves.
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>