[TCP]: Only re-set TSO size for packet which was TSO to begin with.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -588,7 +588,8 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
 	/* Any change of skb->len requires recalculation of tso
 	 * factor and mss.
 	 */
-	tcp_set_skb_tso_segs(skb, tcp_skb_mss(skb));
+	if (tcp_skb_mss(skb))
+		tcp_set_skb_tso_segs(skb, tcp_skb_mss(skb));
 
 	return 0;
 }