nfsd: Get rid of NFSD_VNET macros

When the nfsd was vnet'd, the VNET macros were hidden
behind macros that had the NFSD_ prefix on them.
This was done because, at the time, it was thought
that something other than vnet might be used for this.
That has not happened and probably will not happen,
so this patch replaces these obscuring macros with
the regular vnet ones.

There should be no semantics change caused by
this commit.

Discussed with: bz, glebius
MFC after: 1 month