NVMe: Fix command setup on IO retry

On retry, the req->special is pointing to an already setup IOD, but we
still need to setup the command context and callback, otherwise you'll
see false twice completed errors and leak requests.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>