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)