scsi: core: Add scsi_prot_ref_tag() helper

We are about to remove the request pointer from struct scsi_cmnd and that
will complicate getting to the ref_tag via t10_pi_ref_tag() in the various
drivers. Introduce a helper function to retrieve the reference tag so
drivers will not have to worry about the details.

Link: https://lore.kernel.org/r/20210609033929.3815-2-martin.petersen@oracle.comReviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Message-Id: <20210609033929.3815-2-martin.petersen@oracle.com>

include/scsi/scsi_cmnd.h

@@ -287,6 +287,13 @@ static inline sector_t scsi_get_lba(struct scsi_cmnd *scmd)
 	return blk_rq_pos(scmd->request);
 }
 
+static inline u32 scsi_prot_ref_tag(struct scsi_cmnd *scmd)
+{
+	struct request *rq = blk_mq_rq_from_pdu(scmd);
+
+	return t10_pi_ref_tag(rq);
+}
+
 static inline unsigned int scsi_prot_interval(struct scsi_cmnd *scmd)
 {
 	return scmd->device->sector_size;