net: filter: constify detection of pkt_type_offset

net: filter: constify detection of pkt_type_offset

Currently we have 2 pkt_type_offset functions doing the same thing and
spread across the architecture files. Remove those and replace them
with a PKT_TYPE_OFFSET macro helper which gets the constant value from a
zero sized sk_buff member right in front of the bitfield with offsetof.
This new offset marker does not change size of struct sk_buff.

Cc: Eric Dumazet <[email protected]>
Cc: Markos Chandras <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Signed-off-by: Denis Kirjanov <[email protected]>
Signed-off-by: Hannes Frederic Sowa <[email protected]>
Acked-by: Alexei Starovoitov <[email protected]>
Acked-by: Daniel Borkmann <[email protected]>
Signed-off-by: David S. Miller <[email protected]>