rbd: img_data requests don't own their page array

Move the check into rbd_obj_request_destroy() to avoid use-after-free
on errors in rbd_img_request_fill(..., OBJ_REQUEST_PAGES, ...), where
pages, owned by the caller, gets freed in rbd_img_request_fill().

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