os: explicitly return nil for {rootChmod,rootMkdir,rootRemove} in root_openat.go
It is consistent with same functions in root_noopenat.go.
Change-Id: I81415fd3922101499fcbbdec97e315add0671acb
GitHub-Last-Rev:
3444e8546ee1877feb644202ad2b05379b4b7e74
GitHub-Pull-Request: golang/go#71715
Reviewed-on: https://go-review.googlesource.com/c/go/+/649235
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>