rgw/s3: RGWGetObj still sets attrs on ReadOp error

when `ReadOp::prepare()` fails with `-ERR_NOT_MODIFIED`, it succeeded in
reading the head object for mtime and attrs like etag. make those attrs
available to RGWGetObj so we can still send ETag and other cache-related
response headers

Fixes: https://tracker.ceph.com/issues/45736

Signed-off-by: Casey Bodley <cbodley@redhat.com>

src/rgw/rgw_op.cc

@@ -2283,11 +2283,11 @@ void RGWGetObj::execute(optional_yield y)
   read_op->params.lastmod = &lastmod;
 
   op_ret = read_op->prepare(s->yield, this);
-  if (op_ret < 0)
-    goto done_err;
   version_id = s->object->get_instance();
   s->obj_size = s->object->get_obj_size();
   attrs = s->object->get_attrs();
+  if (op_ret < 0)
+    goto done_err;
 
   /* STAT ops don't need data, and do no i/o */
   if (get_type() == RGW_OP_STAT_OBJ) {