[JFFS2] Remove broken insert_point optimisation in jffs2_add_tn_to_tree()

The original code would remember, during the first pass over the tree,
a suitable place to start the insertion from when we eventually come
to add a new node.

The optimisation was broken, and we sometimes ended up inserting a new
node in the wrong place because we started the insertion from the wrong
point.

Just ditch the optimisation and start the insertion from the root of the
tree, for now. I'll try it again when I'm feeling cleverer.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>