fix rbd_option parse error

There is two bug in the rbd_options parse:
1. The memeber struct thread_data *td of struct rbd_options is filled in the cmdline parse process.So it is usable and cannot be deleted.

2.The busy_poll offset of struct rbd_options is not the "busy_poll" but "client_name"

After fix these two bugs, the fio test then can run.

Signed-off-by: Ketor Meng <d.ketor@gmail.com>