[JFFS2] Remove another bogus optimisation in jffs2_add_tn_to_tree()

We attempted to insert new nodes into the tree by just using
rb_replace_node to let them replace an earlier node which they
completely overlapped. However, that could place the new node into the
wrong place in the tree, since its start could be node only before the
start of the victim, but before the node _before_ the victim in the tree
(if that previous node actually ends _after_ the new node, thus isn't
entirely overlapped and wasn't itself chosen to be the victim).

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