scsi_cmnd: Introduce scsi_transfer_length helper

In case protection information exists on the wire
scsi transports should include it in the transfer
byte count (even if protection information does not
exist in the host memory space). This helper will
compute the total transfer length from the scsi
command data length and protection attributes.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Cc: stable@vger.kernel.org # 3.15+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>