tcp: Fix thinko making the not-shiftable to cover S|R as well

S|R won't result in S if just SACK is received. DSACK is
another story (but it is covered correctly already).
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1481,7 +1481,7 @@ static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
 
 	/* Normally R but no L won't result in plain S */
 	if (!dup_sack &&
-	    (TCP_SKB_CB(skb)->sacked & TCPCB_TAGBITS) == TCPCB_SACKED_RETRANS)
+	    (TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_RETRANS)) == TCPCB_SACKED_RETRANS)
 		goto fallback;
 	if (!skb_can_shift(skb))
 		goto fallback;