pktgen: ndo_start_xmit can return NET_XMIT_xxx values

This updates pktgen so that it does not decrement skb->users
when it receives valid NET_XMIT_xxx values.  These are now
valid return values from ndo_start_xmit in net-next-2.6.
They also indicate the skb has been consumed.

This fixes pktgen to work correctly with vlan devices.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>