eta: Fix compiler warning

eta: Fix compiler warning

gcc 9 complains about directly referencing pointer values in the packed
structure jobs_eta. E.g.:

warning: taking address of packed member of ‘struct jobs_eta’ may
result in an unaligned pointer value [-Waddress-of-packed-member]

Remove this by using a local void pointer for the rate and iops array
references that generate the warning.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>