os: set full name for Roots created with Root.OpenRoot

os: set full name for Roots created with Root.OpenRoot

Set the Name for a Root created within a Root to be the
concatenation of the parent's path and the name used to open the child.

This matches the behavior for files opened within a Root
with Root.Open.

Fixes #73868

Change-Id: Idf4021602ac25556721b7ef6924dec652c7bf4db
Reviewed-on: https://go-review.googlesource.com/c/go/+/698376
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>