smb: fix "Statfs failed: bucket or container name is needed" when mounting

Before this change, if you mounted the root of the smb then it would
give an error on rclone about and periodically in the mount logs:

    Statfs failed: bucket or container name is needed in remote

This fix makes the smb backend return empty usage in this case which
will stop the errors and show the default 1P of free space.

See: https://forum.rclone.org/t/error-statfs-failed-bucket-or-container-name-is-needed-in-remote/39631

backend/smb/smb.go

@@ -447,7 +447,8 @@ func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err e
 func (f *Fs) About(ctx context.Context) (_ *fs.Usage, err error) {
 	share, dir := f.split("/")
 	if share == "" {
-		return nil, fs.ErrorListBucketRequired
+		// Just return empty info rather than an error if called on the root
+		return &fs.Usage{}, nil
 	}
 	dir = f.toSambaPath(dir)