openvswitch: distinguish between the dropped and consumed skb

distinguish between the dropped and consumed skb, not assume the skb
is consumed always

Cc: Thomas Graf <tgraf@noironetworks.com>
Cc: Pravin Shelar <pshelar@nicira.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>