scsi: fas216: Kill scmd->tag

The driver is attempting to allocate a tag internally which is a no-go with
blk-mq. Switch the driver to use the request tag and kill usage of
scmd->tag and scmd->device->current_tag.

[jpg: Change to use scsi_cmd_to_rq()]

Link: https://lore.kernel.org/r/1631696835-136198-2-git-send-email-john.garry@huawei.com
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>