packet: Revert recent header parsing changes.

This reverts commits:

0f75b09c
cbd89acb
c483e026

Amongst other things, it's modifies the SKB header
to pull the ethernet headers off via eth_type_trans()
on the output path which is bogus.

It's causing serious regressions for people.

Signed-off-by: David S. Miller <davem@davemloft.net>