[JFFS2] Build fragtree in reverse order

Instead of building fragtree starting from node with the smallest version
number, start from the highest. This helps to avoid reading and checking
obsolete nodes.

Signed-off-by: Artem B. Bityutskiy <dedekind@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>