cifs: use empty string instead of / as default directory

cifs: use empty string instead of / as default directory

this keeps the mount sources consistent with previous versions
without this patch there is a small regression, which leads to the
storage not being recognized as being mounted on upgrade:
* pvestatd in older version mount the storage with out trailing /
```
//cifsstore/ISO on /mnt/pve/cifsstore type cifs...
```
* the cifs_is_mounted helper does not recognize it as being mounted
(as the source now has a / in the end)
* attempting to mount leads to
```
mount error(16): Device or resource busy
```

noticed after upgrading and having a cifs storage mounted

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>