ipv4: Fix ip-header identification for gso packets.

ip-header id needs to be incremented even if IP_DF flag is set.
This behaviour was changed in commit 490ab081
(IP_GRE: Fix IP-Identification).

Following patch fixes it so that identification is always
incremented.
Reported-by: Cong Wang <amwang@redhat.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ipip.h

@@ -77,15 +77,11 @@ static inline void tunnel_ip_select_ident(struct sk_buff *skb,
 {
 	struct iphdr *iph = ip_hdr(skb);
 
-	if (iph->frag_off & htons(IP_DF))
-		iph->id	= 0;
-	else {
 	/* Use inner packet iph-id if possible. */
 	if (skb->protocol == htons(ETH_P_IP) && old_iph->id)
 		iph->id	= old_iph->id;
 	else
 		__ip_select_ident(iph, dst,
 				  (skb_shinfo(skb)->gso_segs ?: 1) - 1);
-	}
 }
 #endif

net/ipv4/af_inet.c

@@ -1333,7 +1333,6 @@ static struct sk_buff *inet_gso_segment(struct sk_buff *skb,
 				iph->frag_off |= htons(IP_MF);
 			offset += (skb->len - skb->mac_len - iph->ihl * 4);
 		} else  {
-			if (!(iph->frag_off & htons(IP_DF)))
 			iph->id = htons(id++);
 		}
 		iph->tot_len = htons(skb->len - skb->mac_len);