mount: make sure got into MOUNT_DEAD state after a successful umount (#3444)

mount: make sure got into MOUNT_DEAD state after a successful umount (#3444)

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.