[RBTREE] Merge colour and parent fields of struct rb_node.

We only used a single bit for colour information, so having a whole
machine word of space allocated for it was a bit wasteful. Instead,
store it in the lowest bit of the 'parent' pointer, since that was
always going to be aligned anyway.

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