[SCSI] Fix device removal NULL pointer dereference

Use blk_queue_dead() to test whether the queue is dead instead
of !sdev. Since scsi_prep_fn() may be invoked concurrently with
__scsi_remove_device(), keep the queuedata (sdev) pointer in
__scsi_remove_device(). This patch fixes a kernel oops that
can be triggered by USB device removal. See also
http://www.spinics.net/lists/linux-scsi/msg56254.html.

Other changes included in this patch:
- Swap the blk_cleanup_queue() and kfree() calls in
  scsi_host_dev_release() to make that code easier to grasp.
- Remove the queue dead check from scsi_run_queue() since the
  queue state can change anyway at any point in that function
  where the queue lock is not held.
- Remove the queue dead check from the start of scsi_request_fn()
  since it is redundant with the scsi_device_online() check.

Reported-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Reviewed-by: Tejun Heo <tj@kernel.org>
Cc: <stable@kernel.org>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/hosts.c

@@ -290,6 +290,7 @@ static void scsi_host_dev_release(struct device *dev)
 	struct Scsi_Host *shost = dev_to_shost(dev);
 	struct device *parent = dev->parent;
 	struct request_queue *q;
+	void *queuedata;
 
 	scsi_proc_hostdir_rm(shost->hostt);
 
@@ -299,9 +300,9 @@ static void scsi_host_dev_release(struct device *dev)
 		destroy_workqueue(shost->work_q);
 	q = shost->uspace_req_q;
 	if (q) {
-		kfree(q->queuedata);
-		q->queuedata = NULL;
-		scsi_free_queue(q);
+		queuedata = q->queuedata;
+		blk_cleanup_queue(q);
+		kfree(queuedata);
 	}
 
 	scsi_destroy_command_freelist(shost);