correct error of repeatedly adding io_size to off in do_bench_write

Signed-off-by: jiangheng <jiangheng0511@gmail.com>
Merge pull request #2415 from johnkeepmoving/fix-off-error-in-rbd.cc

correct error of repeatedly adding io_size to off in do_bench_write

Reviewed-by: Loic Dachary <loic-201408@dachary.org>