scsi: core: Remove scsi_cmnd.tag

It is never read, so get rid of it.

Link: https://lore.kernel.org/r/1628862553-179450-4-git-send-email-john.garry@huawei.com


Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>