GSO packet headers must be stored in the linear skb segment.
Move tpacket header parsing before sock_alloc_send_skb. The GSO
follow-on patch will later increase the skb linear argument to
sock_alloc_send_skb if needed for large packets.

The header parsing code does not require an allocated skb, so is
safe to move. Later pass to tpacket_fill_skb the computed data
start and length.
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>