Set the mapping size and features earlier in rbd_dev_probe_finish().

Define rbd_dev_mapping_clear() as an inverse for setting those
fields, and use it both in error handling in rbd_dev_image_probe()
and in the final cleanup in rbd_dev_release().  Change the name
of rbd_dev_set_mapping() to of rbd_dev_mapping_set().
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>