tcm_loop: Show address of tpg in configfs

In the past, the scsi_host's number wasn't shown anywhere, user have
to depends on vpg_83 to find the block device, which is also depends
on backstore implementation.

It's better for tcm_loop to provide necessary straightforward
information on locate the block device it created.

This patch would help to locate the block device created by tcm_loop.
The address would be shown at e.g.

  /sys/kernel/config/target/loopback/naa.60014059436855c1/tpgt_1/address

which would looks like "2:0:1", and the lun number can be found at
"/tpgt_1/lun/lun_0".  Altogether they formated the scsi address of
device as "2:0:1:0", which can be used to locate the device easily
through 'lsscsi'.

(Update to >= v4.4-rc1 configfs attribute usage - nab)
Signed-off-by: Sheng Yang <sheng@yasker.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/loopback/tcm_loop.c

@@ -1036,12 +1036,26 @@ static ssize_t tcm_loop_tpg_transport_status_store(struct config_item *item,
 	return -EINVAL;
 }
 
+static ssize_t tcm_loop_tpg_address_show(struct config_item *item,
+					 char *page)
+{
+	struct se_portal_group *se_tpg = to_tpg(item);
+	struct tcm_loop_tpg *tl_tpg = container_of(se_tpg,
+			struct tcm_loop_tpg, tl_se_tpg);
+	struct tcm_loop_hba *tl_hba = tl_tpg->tl_hba;
+
+	return snprintf(page, PAGE_SIZE, "%d:0:%d\n",
+			tl_hba->sh->host_no, tl_tpg->tl_tpgt);
+}
+
 CONFIGFS_ATTR(tcm_loop_tpg_, nexus);
 CONFIGFS_ATTR(tcm_loop_tpg_, transport_status);
+CONFIGFS_ATTR_RO(tcm_loop_tpg_, address);
 
 static struct configfs_attribute *tcm_loop_tpg_attrs[] = {
 	&tcm_loop_tpg_attr_nexus,
 	&tcm_loop_tpg_attr_transport_status,
+	&tcm_loop_tpg_attr_address,
 	NULL,
 };