[TCP]: Break out tcp_snd_test() into it's constituent parts.

tcp_snd_test() does several different things, use inline
functions to express this more clearly.

1) It initializes the TSO count of SKB, if necessary.
2) It performs the Nagle test.
3) It makes sure the congestion window is adhered to.
4) It makes sure SKB fits into the send window.

This cleanup also sets things up so that things like the
available packets in the congestion window does not need
to be calculated multiple times by packet sending loops
such as tcp_write_xmit().
    
Signed-off-by: David S. Miller <davem@davemloft.net>