Add client references

Add client references

We have a use-after-free in the fio_handle_clients() loop.
If we receive a QUIT command, we remove the client in
fio_handle_client(). But fio_handle_clients() doesn't
have a way to detect this, so it checks client->error
after it has potentially been freed.

Add a simple reference to get rid of this problem.

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