linuxkpi: Take const root in read-only radix tree functions

This is a preparation step for a future addition to this file. This is
also closer to what Linux does.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation

(cherry picked from commit cf39b51d36d82214ebdfd03055d321ddd2d85274)
(cherry picked from commit c5445540995fd37c390ad371a2e95d300ee83988)