openvswitch: Fix a double free bug for the sample action

When sample action returns with an error, the skb has already been
freed. This patch fix a bug to make sure we don't free it again.
This bug introduced by commit ccb1352e76cff05 (net: Add Open vSwitch
kernel components.)

Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>

net/openvswitch/actions.c

@@ -551,6 +551,8 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
 
 		case OVS_ACTION_ATTR_SAMPLE:
 			err = sample(dp, skb, a);
+			if (unlikely(err)) /* skb already freed. */
+				return err;
 			break;
 		}