target: Drop left-over se_lun->lun_status

Now that se_portal_group->tpg_lun_hlist is a RCU protected hlist,
go ahead and drop the left-over lun->lun_status usage.

Reported-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>