Before this change, if you mounted the root of the smb then it would
give an error on rclone about and periodically in the mount logs:
Statfs failed: bucket or container name is needed in remote
This fix makes the smb backend return empty usage in this case which
will stop the errors and show the default 1P of free space.
See: https://forum.rclone.org/t/error-statfs-failed-bucket-or-container-name-is-needed-in-remote/39631
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