doc: packet: add minimal TPACKET_V3 example code

Lost in space for a long time, but it finally came back to us from
some ancient code tombs. This patch adds a minimal runnable example
of Linux' packet mmap(2) from Chetan Loke's TPACKET_V3. Special
thanks to David S. Miller, and also Eric Leblond and Victor Julien!

Cc: Eric Leblond <eric@regit.org>
Cc: Victor Julien <victor@inliniac.net>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>