os: make MkdirAll support volume names

os: make MkdirAll support volume names

MkdirAll fails to create directories under root paths using volume
names (e.g. //?/Volume{GUID}/foo). This is because fixRootDirectory
only handle extended length paths using drive letters (e.g. //?/C:/foo).

This CL fixes that issue by also detecting volume names without path
separator.

Updates #22230
Fixes #39785

Change-Id: I813fdc0b968ce71a4297f69245b935558e6cd789
Reviewed-on: https://go-review.googlesource.com/c/go/+/517015
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>