MemMapFs does not handle mode bits

[jxsl13]

I'm kind of testing around with my BackupFs implementation that also utilizes the Afero.Fs interface in order to implement a backup on write mechanism.

While writing my tests I have noticed that either memmapfs.Chmod is not properly working or memmapfs.Stat is not properly working.

Either way I do get a file mode of 0x0 of type os.ModeTemporary when Stat'ing /Lstat'ing paths.