engines/libaio: use fio_memalign() helper for user iocbs

engines/libaio: use fio_memalign() helper for user iocbs

Don't rely on posix_memalign() being there, that's why we have
this helper.

Also ensure that the memory is cleared. This is important, as
we are passing this to the kernel, and we can't rely on our
->prep() clearing everything all the time.

Signed-off-by: Jens Axboe <axboe@kernel.dk>