The only reason rbd_dev is passed to rbd_get_client() is so its
rbd_client field can get assigned. Instead, just return the
rbd_client pointer as a result and have the caller do the
assignment.
Change rbd_put_client() so it takes an rbd_client structure,
so follows the more typical symmetry with rbd_get_client().
Signed-off-by: Alex Elder <elder@inktank.com> Reviewed-by: Josh Durgin <josh.durgin@inktank.com>