[TCP]: Eliminate redundant computations in tcp_write_xmit().

tcp_snd_test() is run for every packet output by a single
call to tcp_write_xmit(), but this is not necessary.

For one, the congestion window space needs to only be
calculated one time, then used throughout the duration
of the loop.

This cleanup also makes experimenting with different TSO
packetization schemes much easier.

Signed-off-by: David S. Miller <davem@davemloft.net>