nfsd: Wrap nfsstatsv1_p in the NFSD_VNET() macro

Commit 7344856e3a6d added a lot of macros that will front end
vnet macros so that nfsd(8) can run in vnet prison.
The nfsstatsv1_p variable got missed. This patch wraps all
uses of nfsstatsv1_p with the NFSD_VNET() macro.
The NFSD_VNET() macro is still a null macro.

MFC after: 3 months