Update S3 flush to not have last part greater than defined chunkSize

[arvind-choudhary-h]

Fixes: #3873

The new implementation ensures no part size exceeds the defined Chunk size. In the screenshot shared, this is before the changes where the size of the last part was 6579438 which is more than the defined 5242880

The following logic is removed which was responsible for adding the last part bytes into the buf

-	if w.pending.Len() > 0 && w.pending.Len() < int(w.driver.ChunkSize) {
-		if _, err := buf.Write(w.pending.data); err != nil {
-			return err
-		}
-		w.pending.Clear()
-	}

[milosgajdos]

Please, read the whole discussion here: #3940

That's a PR that also attempted adding a recursive flush call, just like this one.

[arvind-choudhary-h]

Thanks for the review!
I went through #3940 earlier and this change was not working for me in the latest main branch; probably due to the deprecated MultipartCombineSmallPart value. Let me revisit the design and get back to this.