net: fix error reported on job exit and full residual

This is expected, when one end drops off.

Signed-off-by: Jens Axboe <axboe@fb.com>
engines/net: add socket buffer window size setting

Signed-off-by: Jens Axboe <axboe@fb.com>
engines/net: add TCP_MAXSEG setting (mss)

Signed-off-by: Jens Axboe <axboe@fb.com>