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 0fd97ccf
  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 0fd97ccf code.

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