SMB Mount to backup directory prevents mounting parent directories of share

May also apply to the "duplicity" package and is related to network "mount"s:

To reproduce:
* Set up a writeable SMB share: smb://1.2.3.4/backup
* Create a subdir: smb://1.2.3.4/backup/UbuntuBackup
* Using "deja-dup" -> "Storage location" set:
    Storage location: Network Server
    Network Location: smb://1.2.3.4/backup/UbuntuBackup
    Folder: MyMachineBackup
* REBOOT THE MACHINE (or clear any cached mounts to "smb://1.2.3.4/backup")
* Select "deja-dup" -> "Overview" -> "Back Up Now..."
* Allow the backup to start, but not finish
* Go to Nautilus (file browser) -> "+ Other Locations"
  - At the bottom set "Connect to server" to: smb://1.2.3.4/backup

Result:
* The parent SMB directory "smb://1.2.3.4/backup" is inaccessible because the "UbuntuBackup" subdir is currently mounted
* Can happen at semi-random times if the backup is on a schedule
* This is especially a problem when backing up remotely over a VPN which can take a long time
* Prevents access to all parent folders of the backup folder
* Nautilus will hang, unable to mount the "backup" parent directory
* Also a problem if the "smb://1.2.3.4/backup" folder is bookmarked in nautilus
* This problem is persistent throughout backup process and for a period of time afterwards until the mount to "UbuntuBackup" times out or the backup process exits

Possible solutions:
* Add a configuration option to deja-dup/duplicity to specify the "Share root" and mount that first before accessing the configured "Network Location"
* Have deja-dup or duplicity attempt to mount the parent directory/directories first by starting at the root of the SMB share and walking down the directory tree until a successful mount is made. In the above example, start by attempting to mount "smb://1.2.3.4/backup" first and then "smb://1.2.3.4/backup/UbuntuBackup" (in the case of deeper folder structures, try each one successively)

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: deja-dup 40.7-0ubuntu1
ProcVersionSignature: Ubuntu 5.13.0-30.33~20.04.1-generic 5.13.19
Uname: Linux 5.13.0-30-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27.21
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Tue Mar 1 08:51:47 2022
ExecutablePath: /usr/bin/deja-dup
InstallationDate: Installed on 2021-09-13 (168 days ago)
InstallationMedia: Ubuntu 20.04.3 LTS "Focal Fossa" - Release amd64 (20210819)
ProcEnviron:
 XDG_RUNTIME_DIR=<set>
 SHELL=/bin/bash
 PATH=(custom, user)
 LANG=en_US.UTF-8
SourcePackage: deja-dup
UpgradeStatus: No upgrade log present (probably fresh install)