io_u: use nsec value for buffer scramble

io_u: use nsec value for buffer scramble

Just use the nanosecond value directly, it's pointless to shift
it down and lose 10 bits with of scrambling data.

Fixes: d5d3795c ("io_u: don't do expensive int divide for buffer scramble")
Signed-off-by: Jens Axboe <axboe@kernel.dk>