tcp: fix data delivery rate

Now skb->mstamp_skb is updated later, we also need to call
tcp_rate_skb_sent() after the update is done.

Fixes: 8c72c65b

 ("tcp: update skb->skb_mstamp more carefully")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>