openvswitch: fix a use after free

pskb_may_pull() called by arphdr_ok can change skb->data, so put the arp
setting after arphdr_ok to avoid the use the freed memory

Fixes: 0714812134d7d ("openvswitch: Eliminate memset() from flow_extract.")
Cc: Jesse Gross <jesse@nicira.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/openvswitch/flow.c

@@ -557,10 +557,11 @@ static int key_extract(struct sk_buff *skb, struct sw_flow_key *key)
 	} else if (key->eth.type == htons(ETH_P_ARP) ||
 		   key->eth.type == htons(ETH_P_RARP)) {
 		struct arp_eth_header *arp;
+		bool arp_available = arphdr_ok(skb);
 
 		arp = (struct arp_eth_header *)skb_network_header(skb);
 
-		if (arphdr_ok(skb) &&
+		if (arp_available &&
 		    arp->ar_hrd == htons(ARPHRD_ETHER) &&
 		    arp->ar_pro == htons(ETH_P_IP) &&
 		    arp->ar_hln == ETH_ALEN &&