[#2469] Correctly update indices after incomplete gathering write

 Motivation:
 When doing a gathering write we need to update the indices after the write partial completes. In the current code-base we use the wrong value when compare the expected written bytes and the actual written bytes.

 Modifications:
 Use the correct value when compare.

Result:
Indices are updated correctly and so no corruption can happen when resume writing after data was only partial written before.