tcp: tcp_transmit_skb() optimizations

1) We need to take a timestamp only for skb that should be cloned.

Other skbs are not in write queue and no rtt estimation is done on them.

2) the unlikely() hint is wrong for receivers (they send pure ACK)
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: MF Nowlan <fitz@cs.yale.edu>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-By: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -850,15 +850,15 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
 
 	BUG_ON(!skb || !tcp_skb_pcount(skb));
 
+	if (clone_it) {
+		const struct sk_buff *fclone = skb + 1;
+
 		/* If congestion control is doing timestamping, we must
 		 * take such a timestamp before we potentially clone/copy.
 		 */
 		if (icsk->icsk_ca_ops->flags & TCP_CONG_RTT_STAMP)
 			__net_timestamp(skb);
 
-	if (likely(clone_it)) {
-		const struct sk_buff *fclone = skb + 1;
-
 		if (unlikely(skb->fclone == SKB_FCLONE_ORIG &&
 			     fclone->fclone == SKB_FCLONE_CLONE))
 			NET_INC_STATS_BH(sock_net(sk),