io_u: don't do expensive int divide for buffer scramble

io_u: don't do expensive int divide for buffer scramble

We don't need the conversion from nsec to usec to be exact, so
just shift by 10 instead.

Fixes: 8b6a404cd ("nanosecond: initial commit changing timeval to timespec")
Signed-off-by: Jens Axboe <axboe@kernel.dk>