[SCSI] target: Fix top-level configfs_subsystem default_group shutdown breakage

This patch fixes two bugs uncovered during testing with
slub_debug=FPUZ during module_exit() -> target_core_exit_configfs()
with release of configfs subsystem consumer default groups, namely how
this should be working with
fs/configfs/dir.c:configfs_unregister_subsystem() release logic for
struct config_group->default_group.

The first issue involves configfs_unregister_subsystem() expecting to
walk+drain the top-level subsys->su_group.default_groups directly in
unlink_group(), and not directly from the configfs subsystem consumer
for the top level struct config_group->default_groups.  This patch
drops the walk+drain of subsys->su_group.default_groups from TCM
configfs subsystem consumer code, and moves the top-level
->default_groups kfree() after configfs_unregister_subsystem() has
been called.

The second issue involves calling
core_alua_free_lu_gp(se_global->default_lu_gp) to release the
default_lu_gp->lu_gp_group before configfs_unregister_subsystem() has
been called.  This patches also moves the core_alua_free_lu_gp() call
to release default_lu_group->lu_gp_group after the subsys has been
unregistered.

Finally, this patch explictly clears the
[lu_gp,alua,hba]_cg->default_groups pointers after kfree() to ensure
that no stale memory is picked up from child struct
config_group->default_group[] while configfs_unregister_subsystem() is
called.
Reported-by: Fubo Chen <fubo.chen@gmail.com>
Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>

drivers/target/target_core_configfs.c

@@ -3178,8 +3178,7 @@ static void target_core_exit_configfs(void)
 		config_item_put(item);
 	}
 	kfree(lu_gp_cg->default_groups);
-	core_alua_free_lu_gp(se_global->default_lu_gp);
-	se_global->default_lu_gp = NULL;
+	lu_gp_cg->default_groups = NULL;
 
 	alua_cg = &se_global->alua_group;
 	for (i = 0; alua_cg->default_groups[i]; i++) {
@@ -3188,6 +3187,7 @@ static void target_core_exit_configfs(void)
 		config_item_put(item);
 	}
 	kfree(alua_cg->default_groups);
+	alua_cg->default_groups = NULL;
 
 	hba_cg = &se_global->target_core_hbagroup;
 	for (i = 0; hba_cg->default_groups[i]; i++) {
@@ -3196,15 +3196,17 @@ static void target_core_exit_configfs(void)
 		config_item_put(item);
 	}
 	kfree(hba_cg->default_groups);
-
-	for (i = 0; subsys->su_group.default_groups[i]; i++) {
-		item = &subsys->su_group.default_groups[i]->cg_item;
-		subsys->su_group.default_groups[i] = NULL;
-		config_item_put(item);
-	}
+	hba_cg->default_groups = NULL;
+	/*
+	 * We expect subsys->su_group.default_groups to be released
+	 * by configfs subsystem provider logic..
+	 */
+	configfs_unregister_subsystem(subsys);
 	kfree(subsys->su_group.default_groups);
 
-	configfs_unregister_subsystem(subsys);
+	core_alua_free_lu_gp(se_global->default_lu_gp);
+	se_global->default_lu_gp = NULL;
+
 	printk(KERN_INFO "TARGET_CORE[0]: Released ConfigFS Fabric"
 			" Infrastructure\n");