LinuxKPI: Add rb_add_cached function

rb_add_cached inserts node into the leftmost cached tree

Sponsored by: Serenity CyberSecurity, LLC
MFC after: 1 week
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D45608

(cherry picked from commit a5cac2e672a7d94c0391076e7452d6a487f7b5c1)