NFS: Add a private local dispatcher for NFSv4 callback operations

The client's NFSv4 callback service is the only remaining user of
svc_generic_dispatch().

Note that the NFSv4 callback service doesn't use the .pc_encode and
.pc_decode callouts in any substantial way, so they are removed.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>