make ioengine specific command line options work

If the ioengine specific parameter is given on the command line,
unrelated value in thread_data.o will be corrupted unexpectedly.
Specifically, if libaio specific parameter userspace_reap is given
on the command line, thread_data.o.description will be set to 1.

This fixes it by passing the correct pointer of ioengine specific
options to the command line parser.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>