[SCSI] fix usb storage oops

The problem is that scsi_run_queue is called from scsi_next_command()
after doing a scsi_put_command.  If the command was the only thing
holding the reference on the scsi_device then the resulting device put
will tear down the block queue.  Fix this by taking a reference to the
device and holding it around scsi_run_queue()
Signed-off-by: NJames Bottomley <James.Bottomley@SteelEye.com>

drivers/scsi/scsi_lib.c

@@ -542,10 +542,17 @@ static void scsi_requeue_command(struct request_queue *q, struct scsi_cmnd *cmd)
 
 void scsi_next_command(struct scsi_cmnd *cmd)
 {
-	struct request_queue *q = cmd->device->request_queue;
+	struct scsi_device *sdev = cmd->device;
+	struct request_queue *q = sdev->request_queue;
+
+	/* need to hold a reference on the device before we let go of the cmd */
+	get_device(&sdev->sdev_gendev);
 
 	scsi_put_command(cmd);
 	scsi_run_queue(q);
+
+	/* ok to remove device now */
+	put_device(&sdev->sdev_gendev);
 }
 
 void scsi_run_host_queues(struct Scsi_Host *shost)