target: Move dev_attrib_cit to struct se_subsystem_api

This patch adds support for dev_attrib_cit as an external config_item_type
using TB_CIT_SETUP() helper macro, and sets both ct_item_ops + ct_attr
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_attrib_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 7b84436..735f8567 100644
--- a/include/target/target_core_backend.h
+++ b/include/target/target_core_backend.h
@@ -7,6 +7,7 @@
 
 struct target_backend_cits {
 	struct config_item_type tb_dev_cit;
+	struct config_item_type tb_dev_attrib_cit;
 };
 
 struct se_subsystem_api {