[JFFS2] a way to reduce memory usage

[Dmitry Bazhenov]

> > As I understand, for the moment every reference to raw node is kept in-core,
> > even for obsoleted one. I think it is a bit excessive.
> 
> ... However, we were
> only actually doing that on NOR flash anyway, because on NAND we do need
> access to obsolete dirent nodes.
> 
The algorithm allows to leave in memory only references to those nodes, that
can affect future functionality, but to free references to all other nodes.
The algorithm is appliable for the case of NAND chips also.

--
Dmitry