jffs2: add buffer to cache flash accesses

With this patch JFFS2 code allocates memory buffer of max_totlen size
(size of the largest node, calculated during scan time) and uses it to
store entire node. Speeds up loading. If malloc fails we use old ways
to do things.

Signed-off-by: Alexey Neyman <avn@emcraft.com>
Signed-off-by: Ilya Yanok <yanok@emcraft.com>