iolog: Logging blocksize in IOPS traces

When saving bandwidth's with write_bw_log option, the last column
provides the blocksize used during the run.

When doing the same with write_io_log option, the last column was always
set to 0 which is confusing when reading the log file later.

This patch put both write_bw_log & write_io_log using the same semantic
by reporting the blocksize for every sample.