Add function to OSD to get values without taking the lock.

There are some cases of OSD use where the value is only initialized once
at a point where successive access of the value can be done so safely
without the need to take the lock.

Reviewed by: markj
Obtained from: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D44631

(cherry picked from commit 56b274213024957cea378d27393304ae5cc8ded9)