rgw: allow system users to read SLO parts

multisite data sync relies on fetching the object as the system user

Fixes: http://tracker.ceph.com/issues/19027

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit d50d18c)

Conflicts: auth rework

src/rgw/rgw_op.cc

@@ -723,7 +723,9 @@ int RGWGetObj::read_user_manifest_part(rgw_bucket& bucket,
   if (op_ret < 0)
     return op_ret;
 
-  if (!verify_object_permission(s, bucket_policy, &obj_policy, RGW_PERM_READ)) {
+  if (s->system_request) {
+    ldout(s->cct, 2) << "overriding permissions due to system operation" << dendl;
+  } else if (!verify_object_permission(s, bucket_policy, &obj_policy, RGW_PERM_READ)) {
     return -EPERM;
   }