- Export tcp_trim_head() and call it directly from
  tcp_tso_acked().  This also fixes URG handling.

- Make tcp_trim_head() adjust the skb->truesize of
  the packet and liberate that space from the socket
  send buffer.

- In tcp_current_mss(), limit TSO factor to 1/4 of
  snd_cwnd.  The idea is from John Heffner.
Signed-off-by: David S. Miller <davem@davemloft.net>