Fixing bug for partition claim math logic

Fixing bug where if you had 4 cids and 5 partitions the parts slice would be parts[6:8] based on the current math and it would panice with a slice out of bounds error

Fix has been tested on a test cluster of 1,4,5 machines with 5 partitions and works ok now.
you can reuse the `plen` variable here to be consistent with the code above.