rgw: fix RGWObjManifestRule decoder

Only decode the new field if it is a new struct.

Fixes: #8804
Backport: firefly
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit c4afaf9dabd261853a44b2e08f0911c075c1cd3a)