[JFFS2] Simplify the tree insert code.

It isn't _normal_ that we allow key collision in rbtrees, 
but it does not matter as long as the two nodes with the same
version are together.

Signed-off-by: Artem B. Bityuckiy <dedekind@infradead.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>