tcp: reserve tcp_skb_mss() to tcp stack

tcp_gso_segment() and tcp_gro_receive() are not strictly
part of TCP stack. They should not assume tcp_skb_mss(skb)
is in fact skb_shinfo(skb)->gso_size.

This will allow us to change tcp_skb_mss() in following patches.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_offload.c

@@ -77,7 +77,7 @@ struct sk_buff *tcp_gso_segment(struct sk_buff *skb,
 	oldlen = (u16)~skb->len;
 	__skb_pull(skb, thlen);
 
-	mss = tcp_skb_mss(skb);
+	mss = skb_shinfo(skb)->gso_size;
 	if (unlikely(skb->len <= mss))
 		goto out;
 
@@ -242,7 +242,7 @@ struct sk_buff **tcp_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 		flush |= *(u32 *)((u8 *)th + i) ^
 			 *(u32 *)((u8 *)th2 + i);
 
-	mss = tcp_skb_mss(p);
+	mss = skb_shinfo(p)->gso_size;
 
 	flush |= (len - 1) >= mss;
 	flush |= (ntohl(th2->seq) + skb_gro_len(p)) ^ ntohl(th->seq);