tcp: remove pointless .dsack/.num_sacks code

In the pure assignment case, the earlier zeroing is
still in effect.

David S. Miller raised concerns if the ifs are there to avoid
dirtying cachelines. I came to these conclusions:

> We'll be dirty it anyway (now that I check), the first "real" statement
> in tcp_rcv_established is:
>
>       tp->rx_opt.saw_tstamp = 0;
>
> ...that'll land on the same dword. :-/
>
> I suppose the blocks are there just because they had more complexity
> inside when they had to calculate the eff_sacks too (maybe it would
> have been better to just remove them in that drop-patch so you would
> have had less head-ache :-)).
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -4248,8 +4248,7 @@ static void tcp_sack_remove(struct tcp_sock *tp)
 		this_sack++;
 		sp++;
 	}
-	if (num_sacks != tp->rx_opt.num_sacks)
-		tp->rx_opt.num_sacks = num_sacks;
+	tp->rx_opt.num_sacks = num_sacks;
 }
 
 /* This one checks to see if we can put data from the
@@ -4325,8 +4324,7 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
 
 	TCP_ECN_accept_cwr(tp, skb);
 
-	if (tp->rx_opt.dsack)
-		tp->rx_opt.dsack = 0;
+	tp->rx_opt.dsack = 0;
 
 	/*  Queue data for delivery to the user.
 	 *  Packets in sequence go to the receive queue.
@@ -4445,7 +4443,6 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
 		/* Initial out of order segment, build 1 SACK. */
 		if (tcp_is_sack(tp)) {
 			tp->rx_opt.num_sacks = 1;
-			tp->rx_opt.dsack     = 0;
 			tp->selective_acks[0].start_seq = TCP_SKB_CB(skb)->seq;
 			tp->selective_acks[0].end_seq =
 						TCP_SKB_CB(skb)->end_seq;

net/ipv4/tcp_output.c

@@ -441,8 +441,7 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
 			*ptr++ = htonl(sp[this_sack].end_seq);
 		}
 
-		if (tp->rx_opt.dsack)
-			tp->rx_opt.dsack = 0;
+		tp->rx_opt.dsack = 0;
 	}
 }