NFSD: Clean up nfsd4_encode_getdeviceinfo()

Adopt the conventional XDR utility functions. Also, restructure to
make the function align more closely with the spec -- there doesn't
seem to be a performance need for speciality code, so prioritize
readability.

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