server: fix wrong use of exit() in server when daemonized

We can't use exit(), or we'll trigger the atexit() hook and
free the shared mem used for mutexes, etc. This can cause a
hang when using a daemonized fio server.

Reported-by: Castor Fu <castor@egocast.org>
Signed-off-by: Jens Axboe <axboe@fb.com>