fix a bug of rbd engine when specify invalidate=1 or fadvise_hint=1.

When specify invalidate=1 or fadvise_hint=1, fio will call posix_fadvise(2), which will return
-EBADF and prevent fio librbd engine acting normally.

We set file type to FIO_TYPE_CHAR in fio_rbd_open() to work-around this problem.