fixing "umount -f" for the NFS client

[Mark Felder]

On Thu, Aug 29, 2013, at 7:50, Rick Macklem wrote:
> 
> Finally, neither of these options are safe to use for NFSv4, because
> operations that manipulate state (like locks) cannot be safely
> interrupted,
> since they will leave the lock in an undefined state, at least for the
> FreeBSD server.
> So, at least for NFSv4, a forced dismount is a necessary alternative.
> 

I was not aware of this specific issue, so thank you for the warning