target: Fix se_tpg_tfo->tf_subsys regression + remove tf_subsystem

There is just one configfs subsystem in the target code, so we might as
well add two helpers to reference / unreference it from the core code
instead of passing pointers to it around.

This fixes a regression introduced for v4.1-rc1 with commit 9ac8928e6,
where configfs_depend_item() callers using se_tpg_tfo->tf_subsys would
fail, because the assignment from the original target_core_subsystem[]
is no longer happening at target_register_template() time.

(Fix target_core_exit_configfs pointer dereference - Sagi)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Himanshu Madhani <himanshu.madhani@qlogic.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
index 5e19bb5..ea32b38 100644
--- a/drivers/vhost/scsi.c
+++ b/drivers/vhost/scsi.c
@@ -1409,8 +1409,7 @@
 			 * dependency now.
 			 */
 			se_tpg = &tpg->se_tpg;
-			ret = configfs_depend_item(se_tpg->se_tpg_tfo->tf_subsys,
-						   &se_tpg->tpg_group.cg_item);
+			ret = target_depend_item(&se_tpg->tpg_group.cg_item);
 			if (ret) {
 				pr_warn("configfs_depend_item() failed: %d\n", ret);
 				kfree(vs_tpg);
@@ -1513,8 +1512,7 @@
 		 * to allow vhost-scsi WWPN se_tpg->tpg_group shutdown to occur.
 		 */
 		se_tpg = &tpg->se_tpg;
-		configfs_undepend_item(se_tpg->se_tpg_tfo->tf_subsys,
-				       &se_tpg->tpg_group.cg_item);
+		target_undepend_item(&se_tpg->tpg_group.cg_item);
 	}
 	if (match) {
 		for (i = 0; i < VHOST_SCSI_MAX_VQ; i++) {