target: use idr for se_device dev index

In the next patches we will add tcmu netlink support that allows
userspace to send commands to target_core_user. To execute operations
on a se_device/tcmu_dev we need to be able to look up a dev by any old
id. This patch replaces the se_device->dev_index with a idr created
id.

The next patches will also remove the g_device_list and replace it with
the idr.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

include/target/target_core_base.h

@@ -219,7 +219,6 @@ enum tcm_tmrsp_table {
  */
 typedef enum {
 	SCSI_INST_INDEX,
-	SCSI_DEVICE_INDEX,
 	SCSI_AUTH_INTR_INDEX,
 	SCSI_INDEX_TYPE_MAX
 } scsi_index_t;