This makes `systemd-umount` or `systemd-mount -u` support unmounting loop devices by the corresponding backing files, like `systemd-mount --umount /tmp/foo.img /tmp/bar.img` Fixes #6206.
This makes `systemd-umount` or `systemd-mount -u` support unmounting loop devices by the corresponding backing files, like `systemd-mount --umount /tmp/foo.img /tmp/bar.img` Fixes #6206.