gro: Use gso_size to store MSS

In order to allow GRO packets without frag_list at all, we need to
store the MSS in the packet itself.  The obvious place is gso_size.
The only thing to watch out for is if the packet ends up not being
GRO then we need to clear gso_size before pushing the packet into
the stack.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2365,6 +2365,7 @@ static int napi_gro_complete(struct sk_buff *skb)
 	}
 
 out:
+	skb_shinfo(skb)->gso_size = 0;
 	__skb_push(skb, -skb_network_offset(skb));
 	return netif_receive_skb(skb);
 }
@@ -2446,6 +2447,7 @@ int napi_gro_receive(struct napi_struct *napi, struct sk_buff *skb)
 	}
 
 	NAPI_GRO_CB(skb)->count = 1;
+	skb_shinfo(skb)->gso_size = skb->len;
 	skb->next = napi->gro_list;
 	napi->gro_list = skb;
 

net/core/skbuff.c

@@ -2613,6 +2613,7 @@ int skb_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 
 	*NAPI_GRO_CB(nskb) = *NAPI_GRO_CB(p);
 	skb_shinfo(nskb)->frag_list = p;
+	skb_shinfo(nskb)->gso_size = skb_shinfo(p)->gso_size;
 	skb_header_release(p);
 	nskb->prev = p;
 

net/ipv4/tcp.c

@@ -2519,9 +2519,7 @@ struct sk_buff **tcp_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 	flush |= memcmp(th + 1, th2 + 1, thlen - sizeof(*th));
 
 	total = p->len;
-	mss = total;
-	if (skb_shinfo(p)->frag_list)
-		mss = skb_shinfo(p)->frag_list->len;
+	mss = skb_shinfo(p)->gso_size;
 
 	flush |= skb->len > mss || skb->len <= 0;
 	flush |= ntohl(th2->seq) + total != ntohl(th->seq);
@@ -2557,7 +2555,6 @@ int tcp_gro_complete(struct sk_buff *skb)
 	skb->csum_offset = offsetof(struct tcphdr, check);
 	skb->ip_summed = CHECKSUM_PARTIAL;
 
-	skb_shinfo(skb)->gso_size = skb_shinfo(skb)->frag_list->len;
 	skb_shinfo(skb)->gso_segs = NAPI_GRO_CB(skb)->count;
 
 	if (th->cwr)