NFSD: Clean up nfsd4_encode_exchange_id()

Restructure nfsd4_encode_exchange_id() so that it will be more
straightforward to add support for SSV one day. Also, adopt the use
of the conventional XDR utility functions.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>