target: Drop unnecessary core_tpg_register TFO parameter

This patch drops unnecessary target_core_fabric_ops parameter usage
for core_tpg_register() during fabric driver TFO->fabric_make_tpg()
se_portal_group creation callback execution.

Instead, use the existing se_wwn->wwn_tf->tf_ops pointer to ensure
fabric driver is really using the same TFO provided at module_init
time.

Also go ahead and drop the forward TFO declarations tree-wide, and
handling the special case for iscsi-target discovery TPG.

Cc: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
diff --git a/drivers/xen/xen-scsiback.c b/drivers/xen/xen-scsiback.c
index 555033b..25144a0 100644
--- a/drivers/xen/xen-scsiback.c
+++ b/drivers/xen/xen-scsiback.c
@@ -203,8 +203,6 @@
 static DEFINE_MUTEX(scsiback_mutex);
 static LIST_HEAD(scsiback_list);
 
-static const struct target_core_fabric_ops scsiback_ops;
-
 static void scsiback_get(struct vscsibk_info *info)
 {
 	atomic_inc(&info->nr_unreplied_reqs);
@@ -1765,8 +1763,7 @@
 	tpg->tport = tport;
 	tpg->tport_tpgt = tpgt;
 
-	ret = core_tpg_register(&scsiback_ops, wwn, &tpg->se_tpg,
-				tport->tport_proto_id);
+	ret = core_tpg_register(wwn, &tpg->se_tpg, tport->tport_proto_id);
 	if (ret < 0) {
 		kfree(tpg);
 		return NULL;