skd_main: don't use req->special

Add a retries field to the internal request structure instead, which gets
set to zero on the first submission.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>