[SCSI] sd: Correctly handle 6-byte commands with DIX

DIF does not work with 6-byte commands so we previously ignored those
commands when preparing a request.  However, DIX does not need
RDPROTECT/WRPROTECT to be set and 6-byte commands are consequently
perfectly valid in host-only mode.

This patch fixes a problem where we would set the wrong DIX operation
when issuing commands to a legacy disk.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/sd_dif.c

@@ -374,7 +374,10 @@ void sd_dif_op(struct scsi_cmnd *scmd, unsigned int dif, unsigned int dix, unsig
 	else
 		csum_convert = 0;
 
+	BUG_ON(dif && (scmd->cmnd[0] == READ_6 || scmd->cmnd[0] == WRITE_6));
+
 	switch (scmd->cmnd[0]) {
+	case READ_6:
 	case READ_10:
 	case READ_12:
 	case READ_16:
@@ -390,6 +393,7 @@ void sd_dif_op(struct scsi_cmnd *scmd, unsigned int dif, unsigned int dix, unsig
 
 		break;
 
+	case WRITE_6:
 	case WRITE_10:
 	case WRITE_12:
 	case WRITE_16: