NFSD: Clean up nfsd4_encode_layoutget()

De-duplicate the open-coded stateid4 encoder. Adopt the use of the
conventional current XDR encoding helpers. Refactor the encoder to
align with the XDR specification.

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