Update benchmarks/fio to 3.5
I've found at least one, probably two, significant bugs in fio-3.0 (Aug 2017) on FreeNAS 11.1 that can be alleviated by updating to a newer version. FreeNAS 11.2 includes fio-3.5 (Feb 2018) which fixes the issues I have encountered.
The major issue was that the rw=randrw setting does not generate random I/O properly - it isn't truly random.
I also believe that the logged statistics (write_bw_log, write_lat_log, etc.) are not accurate with fio-3.0.
Update version of fio in line with 11.2-stable
Resolves some significant bugs found in fio 3.0