rbd: rework rbd_img_obj_exists_submit() error paths

- don't put obj_request before rbd_obj_request_get() if
  rbd_obj_request_create() fails
- don't leak pages if rbd_obj_request_create() fails
- don't leak stat_request if rbd_osd_req_create() fails

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