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>

include/scsi/scsi_host.h

@@ -421,6 +421,11 @@ struct scsi_host_template {
 	 */
 	unsigned char present;
 
+	/*
+	 * Let the block layer assigns tags to all commands.
+	 */
+	unsigned use_blk_tags:1;
+
 	/*
 	 * This specifies the mode that a LLD supports.
 	 */