[TCP]: Comment fastpath_cnt_hint off-by-one trap

Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -343,7 +343,8 @@ struct tcp_sock {
 	struct sk_buff *forward_skb_hint;
 	struct sk_buff *fastpath_skb_hint;
 
-	int     fastpath_cnt_hint;
+	int     fastpath_cnt_hint;	/* Lags behind by current skb's pcount
+					 * compared to respective fackets_out */
 	int     lost_cnt_hint;
 	int     retransmit_cnt_hint;