[TCP]: Update comment about highest_sack validity

This stale info came from the original idea, which proved to be
unnecessarily complex, sacked_out > 0 is easy to do and that when
it's going to be needed anyway (it _can_ be valid also when
sacked_out == 0 but there's not going to be a guarantee about it
for now).
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -332,7 +332,8 @@ struct tcp_sock {
 
 	struct tcp_sack_block_wire recv_sack_cache[4];
 
-	u32	highest_sack;	/* Start seq of globally highest revd SACK (valid only in slowpath) */
+	u32	highest_sack;	/* Start seq of globally highest revd SACK
+				 * (validity guaranteed only if sacked_out > 0) */
 
 	/* from STCP, retrans queue hinting */
 	struct sk_buff* lost_skb_hint;