Define RB_SET_PARENT to do all assignments to rb parent
pointers. Define RB_SWAP_CHILD to replace the child of a parent with
its twin, and use it in 4 places. Use RB_SET in rb_link_node to remove
the only linuxkpi reference to color, and then drop color- and
parent-related definitions that are defined and used only in rbtree.h.

This is intended to be entirely cosmetic, with no impact on program
behavior, and leave RB_PARENT and RB_SET_PARENT as the only ways to
read and write rb parent pointers.

Reviewed by: markj, kib
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D25264