os: fix Root.MkdirAll to handle race of directory creation
No tests were added, because in order to reproduce, the directory would
have to be created precisely between the rootOpenDir and mkdirat calls,
which is impossible to do in a test.
Fixes #75114
Change-Id: I6f86a5b33c87452c35728318eaf2169a7534ef37
Reviewed-on: https://go-review.googlesource.com/c/go/+/698215
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Sean Liao <sean@liao.dev> Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Sean Liao <sean@liao.dev>