sharing: make sure that we only find the shares from a given owner 

Make sure that we only find the shares from a given owner and don't expose shares from other users.

How to test:

- create three user: user1, user2, user3, user4
- user1 creates following folder structure: /folder/subfolder/subsubfolder
- user1 shares folder to user2 and to user3
- login as user2
- user2 shares folder/subfolder to user4
- user2 open share drop down for folder/subfolder/subsubfolder

Expected Result:
The drop down shows 
```
Shared in subfolder with user4
```

Result before this patch:
The drop down shows
```
Shared in folder with user2, user3
Shared in subfolder with user4
```
Note: you can also use `$this->assertEmpty()` in the future :smile: