target: Move dev_stat_cit to struct se_subsystem_api

This patch adds support for dev_stat_cit as an external config_item_type
using TB_CIT_SETUP() helper macro, and sets only ct_group_ops following
existing code.

It updates target_core_make_subdev() + target_core_setup_dev_attrib_cit() +
struct target_backend_cits, and drops left-over target_core_dev_stat_cit
from target_core_configfs.c code and update comments.

This patch introduces no functional change from existing code.

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
diff --git a/include/target/target_core_backend.h b/include/target/target_core_backend.h
index 157d173..83a3726 100644
--- a/include/target/target_core_backend.h
+++ b/include/target/target_core_backend.h
@@ -11,6 +11,7 @@
 	struct config_item_type tb_dev_pr_cit;
 	struct config_item_type tb_dev_wwn_cit;
 	struct config_item_type tb_dev_alua_tg_pt_gps_cit;
+	struct config_item_type tb_dev_stat_cit;
 };
 
 struct se_subsystem_api {