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.

MFC after: 1 week
Sponsored by: Axcient
Reviewed by: rmacklem, ken
Differential Revision: https://reviews.freebsd.org/D37589