rbd: don't crash or leak on errors in rbd_img_obj_parent_read_full_callback()

- fix parent_length == img_request->xferred assert to not fire on
  copyup read failures
- don't leak pages if copyup read fails or we can't allocate a new osd
  request

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