Allow any user to read the NFS stats, for example with nfsstat(1).

This was originally allowed by 3cea29603d3 (2011). But it got broken by
693957f8861 (2016) and apparently nobody noticed.

Sponsored by: Axcient
Reviewed by: rmacklem, ken
Differential Revision: https://reviews.freebsd.org/D37589

(cherry picked from commit d2ce00e9a6176014bbeb792dd9959ef1e60d787e)