rgw: use s->bucket_attrs instead of trying to read obj attrs

rgw: use s->bucket_attrs instead of trying to read obj attrs

Fixes: #10307
Backport: firefly, giant

This is needed, since we can't really read the bucket attrs by trying to
read the bucket entry point attrs. We already have the bucket attrs
anyway, use these.

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