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.com


Reviewed-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>