Network Drop Monitor: Fix skb_kill_datagram

Commit ead2ceb0

 ("Network Drop Monitor:
Adding kfree_skb_clean for non-drops and modifying end-of-line points
for skbs") established new conventions for identifying dropped packets.

Align skb_kill_datagram() with these conventions so that packets that
get dropped just before the copy to userspace are properly tracked.

Signed-off-by: John Dykstra <john.dykstra1@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>