refcount: Constify refcount_load()
Reviewed by: markj
Approved by: markj (mentor)
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46888
(cherry picked from commit 9dc5b3dbb063f55543ff8a0619eee2e294fb7dc5)