fix net engine client read server write bug

fix net engine client read server write bug

The net engine was updated with specific 'listen' parameter which used
for identifying whether it is a tcp server, instead of previous
'rw=read' side must be the server. This let fio support bi-direction
net io test. fio_netio_open_file() has to follow this
principle. Otherwise, the following tcp client 'rw=read' and tcp
server 'rw=write' case doesn't work.

[global]
ioengine=net
port=8888
protocol=tcp
bs=4k
size=10g

[server]
listen
rw=write

[client]
hostname=localhost
startdelay=1
rw=read

BTW, examples/netio needs 'listen' and 'hostname' adjusted.

Signed-off-by: Jens Axboe <axboe@kernel.dk>