rbd: mark the original request as done if stat request fails

If stat request fails with something other than -ENOENT (which just
means that we need to copyup), the original object request is never
marked as done and therefore never completed.  Fix this by moving the
mark done + complete snippet from rbd_img_obj_parent_read_full() into
rbd_img_obj_exists_callback().  The former remains covered, as the
latter is its only caller (through rbd_img_obj_request_submit()).

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: David Disseldorp <ddiss@suse.de>