blk-mq: get rid of ->cmd_size in the hardware queue

We store it in the tag set, we don't need it in the hardware queue.
While removing cmd_size, place ->queue_num further down to avoid
a hole on 64-bit archs. It's not used in any fast paths, so we
can safely move it.

Signed-off-by: Jens Axboe <axboe@fb.com>