don't move a share mount point into a different mount point

fix for #8429 

Don't move a share mount point into another mount point.

cc @PVince81 @icewind1991 
intuitively I would expect that the "Home storage" is always the home/root storage. Alternative we could first ask for the storage of the root and then check if both are from the same class, something like:

```
list($rootStorage,) = \OC\Files\Filesystem::resolvePath('/');
list($targetStorage,) = \OC\Files\Filesystem::resolvePath($target);
if ($rootStorage == $targetStorage) {
  ...
```