Stable6 fix encryption

Should fix rename of encryption keys if sub-subfolder doesn't exists.

see #6411 

Maybe it also fix #8936 
> Okay. I also assume that we don't want/need to trigger hooks here
> since we work inside of "datadir/$user/files_encryption" directly ?

Yes, as soon as we support recursive mkdir in \OC\Files\View we can use
the view again, afaik there is a pull request for it