TARGET/sbc,loopback: Adjust command data length in case pi exists on the wire

In various areas of the code, it is assumed that
se_cmd->data_length describes pure data. In case
that protection information exists over the wire
(protect bits is are on) the target core re-calculates
the data length from the CDB and the backed device
block size (instead of each transport peeking in the cdb).

Modify loopback device to include protection information
in the transferred data length (like other scsi transports).

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