NFSD: Add support for NFS v4.2 operation checking

The server does allow NFS over v4.2, even if it doesn't add any new
operations yet.

I also switch to using constants to represent the last operation for
each minor version since this makes the code cleaner and easier to
understand at a quick glance.

Signed-off-by: Anna Schumaker <bjschuma@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>