rb_tree: pass parent to RB_INSERT_COLOR

Change RB_COLOR_INSERT to take a parent parameter, to avoid looking up
a value already available. Make adjustments to a linux rbtree header,
which invokes it.

Reviewed by: alc, hselasky
Differential Revision: https://reviews.freebsd.org/D36114

(cherry picked from commit 4893472c9a18cd8ce3b68d0c54084ef6f0285d0f)