daemon/graphdriver/windows: use go-winio.GetFileSystemType()

go-winio now defines this function, so we can consume that.

Note that there's a difference between the old implementation and the original
one (added in 1cb9e9b44e). The old implementation
had special handling for win32 error codes, which was removed in the go-winio
implementation in 0966e1ad56

As `go-winio.GetFileSystemType()` calls `filepath.VolumeName(path)` internally,
this patch also removes the `string(home[0])`, which is redundant, and could
potentially panic if an empty string would be passed.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

vendor/github.com/Microsoft/go-winio/pkg/fs/fs_windows.go

@@ -0,0 +1,31 @@
+package fs
+
+import (
+	"errors"
+	"path/filepath"
+
+	"golang.org/x/sys/windows"
+)
+
+var (
+	// ErrInvalidPath is returned when the location of a file path doesn't begin with a driver letter.
+	ErrInvalidPath = errors.New("the path provided to GetFileSystemType must start with a drive letter")
+)
+
+// GetFileSystemType obtains the type of a file system through GetVolumeInformation.
+// https://msdn.microsoft.com/en-us/library/windows/desktop/aa364993(v=vs.85).aspx
+func GetFileSystemType(path string) (fsType string, err error) {
+	drive := filepath.VolumeName(path)
+	if len(drive) != 2 {
+		return "", ErrInvalidPath
+	}
+
+	var (
+		buf  = make([]uint16, 255)
+		size = uint32(windows.MAX_PATH + 1)
+	)
+	drive += `\`
+	err = windows.GetVolumeInformation(windows.StringToUTF16Ptr(drive), nil, 0, nil, nil, nil, &buf[0], size)
+	fsType = windows.UTF16ToString(buf)
+	return
+}