NFSD: Add generic v4.2 infrastructure

It's cleaner to introduce everything at once and have the server reply
with "not supported" than it would be to introduce extra operations when
implementing a specific one in the middle of the list.

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