NFS: Update xdr_encode_foo() functions that we're keeping

Clean up.  Move the timestamp and the sattr encoder to match the
placement convention of the other helpers, update their coding style,
and refresh their documenting comments.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>