sd: rename the scsi_disk.dev field

dev is very hard to grep for.  Give the field a more descriptive name and
documents its purpose.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20220308055200.735835-5-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>