[sharing] return the new name if a mount point was renamed

we should update the "name" of the file/folder to reflect the current mount point. Otherwise the filename will always fall back to the owners filename if a user renames a shared file/folder.

Steps to test:

- user1 shares a folder "folder" with user2
- user2 renames "folder" to "folder2"
- without this pr: after the rename was finished successfully the name in the web interface will still be "folder" until you reload the page
- with this pr: you will see the new name immediately

cc @icewind1991 please also have a look because this touches the file cache behavior.