[SCSI] export device_busy for sdev

If you mutiple devices connect to a host, we might be interested in
have an intensive I/O workload on one disk, and notice starvation on others.
This give the user more hint about current infight io for scsi device.
Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/scsi_sysfs.c

@@ -529,6 +529,7 @@ static int scsi_sdev_check_buf_bit(const char *buf)
  */
 sdev_rd_attr (device_blocked, "%d\n");
 sdev_rd_attr (queue_depth, "%d\n");
+sdev_rd_attr (device_busy, "%d\n");
 sdev_rd_attr (type, "%d\n");
 sdev_rd_attr (scsi_level, "%d\n");
 sdev_rd_attr (vendor, "%.8s\n");
@@ -750,6 +751,7 @@ static struct attribute *scsi_sdev_attrs[] = {
 	&dev_attr_device_blocked.attr,
 	&dev_attr_type.attr,
 	&dev_attr_scsi_level.attr,
+	&dev_attr_device_busy.attr,
 	&dev_attr_vendor.attr,
 	&dev_attr_model.attr,
 	&dev_attr_rev.attr,