Without this code the following can happen:
1. Open a file to keep a mount busy
2. Try to stop the corresponding mount unit with systemctl
-> umount fails and the failure is remembered in mount->result
3. Close the file and umount the filesystem manually
-> mount_dispatch_io() calls "mount_enter_dead(mount, MOUNT_SUCCESS)"
-> Old error in mount->result is reused and the mount unit enters a
failed state
Clear the old error result when 'mountinfo' reports a successful umount to
fix this.
/* This has just been unmounted by
* somebody else, follow the state
* change. */
+ mount->result = MOUNT_SUCCESS;
mount_enter_dead(mount, MOUNT_SUCCESS);
break;