tcp: use tcp_skb_sent_after() instead in RACK

This patch doesn't change any functionality.

Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Tested-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>