target: Fix regression allowing unconfigured devices to fabric port link

This patch fixes a v3.8-rc1 regression bug where an unconfigured se_device
was incorrectly allowed to perform a fabric port-link.  This bug was
introduced in commit:

  commit 0fd97cc
  Author: Christoph Hellwig <hch@infradead.org>
  Date:   Mon Oct 8 00:03:19 2012 -0400

      target: kill struct se_subsystem_dev

which ended up dropping the original se_subsystem_dev->se_dev_ptr check
preventing this from happening with pre commit 0fd97cc code.

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_fabric_configfs.c

@@ -754,6 +754,11 @@ static int target_fabric_port_link(
 		return -EFAULT;
 	}
 
+	if (!(dev->dev_flags & DF_CONFIGURED)) {
+		pr_err("se_device not configured yet, cannot port link\n");
+		return -ENODEV;
+	}
+
 	tpg_ci = &lun_ci->ci_parent->ci_group->cg_item;
 	se_tpg = container_of(to_config_group(tpg_ci),
 				struct se_portal_group, tpg_group);