target: Move dev_wwn_cit to struct se_subsystem_api

target: Move dev_wwn_cit to struct se_subsystem_api

This patch adds support for dev_wwn_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_wwn_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>