[patch] Adding Secure Deletion to UBIFS

[Joel Reardon]

>
> I think you can use them. But is it possible to kill these things from
> the data nodes themselves? We can always find it by looking up the index
> by the data node key, right?
>

Yep, thats exactly right. Originally it was just inside the data node;
I put it into the TNC as a optimization later during development. If I use
the last eight bytes from the data node's key, and I remove the position
from ubifs_branch, then there is no longer any change to the on-disk
format, so thats should be useful. Yeah, storing the same value twice
means also needing a consistency check. What can be done is that when the
value is loaded once after mounting, it is stored in in the z-node in
memory.