rb_tree: silence coverity

Add comments to RB_INSERT_COLOR to silence coverity warnings about the
use of an uninitialized variable. Since other static analyzers will
complain too, add a comment to explain why the complaints are unwarranted.

Reviewed by: markj
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D35671

(cherry picked from commit 91b30f7ad22642979b56a56b211843cbd9d35984)