rgw: don't use s->bucket for metadata api path entry

rgw: don't use s->bucket for metadata api path entry

Fixes #14549

s->bucket is only initialized at the object store handler (post auth
callback). We don't want to initialize it for other cases because
it's not a real bucket, and initializing it will require going through
the authorization path for this 'bucket'.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>