[SCSI] sd: Issue correct protection operation

Use the same logic to prepare RD/WRPROTECT and the protection
operation.  Fixes a corner case where we could issue an unprotected
CDB and yet tell the HBA to do DIF to the drive.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>