packet: set transport header before doing xmit

packet: set transport header before doing xmit

Set the transport header for 1) some drivers (e.g ixgbe needs l4 header to do
atr) 2) precise packet length estimation (introduced in b8b60fd7) needs l4
header to compute header length.

So this patch first tries to get l4 header for packet socket through
skb_flow_dissect(), and pretend no l4 header if skb_flow_dissect() fails.

Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>