tools/rados: fix "Floating point exception" if objectsize smaller than blocksize...

tools/rados: fix "Floating point exception" if  objectsize smaller than blocksize when do write-bench.

When do write-bench, if objectsize is smaller than blocksize. It will
casue "Floating point exception".
This because in bjBencher::write_bench:
>>if (data.op_size)
>>    writes_per_object = data.object_size / data.op_size;
  It make writes_per_object = 0;
......
>> name[i] = generate_object_name(i / writes_per_object);
  i/0 cause Floating point exception.

If objectsize is smaller than blocksize, make blocksize is eqaul
objectsize.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>