tools: selftests: psock_tpacket: get rid of macro wrappers

The TPACKET_V3 test code consists of a lot of unecessary macro
wrappers that rather obfuscate what members are accessed in what
way. So get rid of them and make the code more readable. Also
credit Chetan for providing tpacket_v3 example code. Furthermore,
get rid of private offset usage, as we do not need it here.

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