Fix integer overflow in rate_iops

Fix integer overflow in rate_iops

For the following job file:

[bla]
rw=randread
bs=1024m
rate_iops=100

We end up overflowing in multiplication, making the bps field 0.
Fio exits by reporting:

rate lower than supported

Fix this by casting to uint64_t, the type of the output.

Signed-off-by: Jens Axboe <axboe@fb.com>