tcp: fix comment for tp->highest_sack

There was an off-by-one error in the comments describing the
highest_sack field in struct tcp_sock. The comments previously claimed
that it was the "start sequence of the highest skb with SACKed
bit". This commit fixes the comments to note that it is the "start
sequence of the skb just *after* the highest skb with SACKed bit".

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 42c29bf..2d80c29 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1364,8 +1364,9 @@
 	}
 }
 
-/* Start sequence of the highest skb with SACKed bit, valid only if
- * sacked > 0 or when the caller has ensured validity by itself.
+/* Start sequence of the skb just after the highest skb with SACKed
+ * bit, valid only if sacked_out > 0 or when the caller has ensured
+ * validity by itself.
  */
 static inline u32 tcp_highest_sack_seq(struct tcp_sock *tp)
 {