packet: minor: add generic tpacket_uhdr to access packet headers

There is no need to add a dozen unions each time at the start
of the function. So, do this once and use it instead. Thus, we
can remove some duplicate code and make it more readable.

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