nvme: move the retries count to struct nvme_request

The way NVMe uses this field is entirely different from the older
SCSI/BLOCK_PC usage, so move it into struct nvme_request.

Also reduce the size of the file to a unsigned char so that we leave
space for additional smaller fields that will appear soon.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@fb.com>