doc: packet: simplify tpacket example code

This patch simplifies the tpacket_v3 example code a bit by getting rid
of unecessary macro wrappers, removing some debugging code so that it is
more to the point, and also adds a header comment. Now this example code
is the very minimum one needs to start from when dealing with tpacket_v3
and ~100 lines smaller than before.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>