tcm_loop: Set residual field for SCSI commands

If the target core signals an over- or under-run, tcm_loop should call
scsi_set_resid() to tell the SCSI midlayer about the residual data length.

The difference can be seen by doing something like

    strace -eioctl sg_raw -r 1024 /dev/sda 8 0 0 0 1 0 > /dev/null

and looking at the "resid=" part of the SG_IO ioctl -- after this patch,
the field is correctly reported as 512.
Signed-off-by: Roland Dreier <roland@purestorage.com>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/loopback/tcm_loop.c

@@ -829,6 +829,9 @@ static int tcm_loop_queue_data_in(struct se_cmd *se_cmd)
 
 	sc->result = SAM_STAT_GOOD;
 	set_host_byte(sc, DID_OK);
+	if ((se_cmd->se_cmd_flags & SCF_OVERFLOW_BIT) ||
+	    (se_cmd->se_cmd_flags & SCF_UNDERFLOW_BIT))
+		scsi_set_resid(sc, se_cmd->residual_count);
 	sc->scsi_done(sc);
 	return 0;
 }
@@ -854,6 +857,9 @@ static int tcm_loop_queue_status(struct se_cmd *se_cmd)
 		sc->result = se_cmd->scsi_status;
 
 	set_host_byte(sc, DID_OK);
+	if ((se_cmd->se_cmd_flags & SCF_OVERFLOW_BIT) ||
+	    (se_cmd->se_cmd_flags & SCF_UNDERFLOW_BIT))
+		scsi_set_resid(sc, se_cmd->residual_count);
 	sc->scsi_done(sc);
 	return 0;
 }