scsi: always assign block layer tags if enabled

Allow a driver to ask for block layer tags by setting .use_blk_tags in the
host template, in which case it will always see a valid value in
request->tag, similar to the behavior when using blk-mq.  This means even
SCSI "untagged" commands will now have a tag, which is especially useful
when using a host-wide tag map.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>

drivers/message/fusion/mptsas.c

@@ -1994,6 +1994,7 @@ static struct scsi_host_template mptsas_driver_template = {
 	.cmd_per_lun			= 7,
 	.use_clustering			= ENABLE_CLUSTERING,
 	.shost_attrs			= mptscsih_host_attrs,
+	.use_blk_tags			= 1,
 };
 
 static int mptsas_get_linkerrors(struct sas_phy *phy)