[PATCH] make the SCSI mid-layer obey the device online flag

It has been pointed out by the USB people that the mid-layer doesn't
obey its own online flag.

The attached patch should fix this.  However, there are a few caveats to
offlining (read that as devices should still be prepared to process
commands).

1. Any special command will still be accepted (that's a command either
via the SCSI_IOCTL_SEND_COMMAND, or an internally generated command).
2. Outstanding already processed commands in the queue (i.e. commands
which have already been through the upper layer drivers but needed
requeuing for some reason like QUEUE_FULL or device busy).

I'm willing to consider changing 2., it just requires more speciallised
logic to distinguish between a command that has been prepared by the
upper level drivers and a command sent via 1.

However, not that LLDs may not assume they will receive no commands just
because scsi_device->online is zero.

drivers/scsi/scsi_lib.c

@@ -944,6 +944,18 @@ static int scsi_prep_fn(struct request_queue *q, struct request *req)
 		} else
 			cmd = req->special;
 	} else if (req->flags & (REQ_CMD | REQ_BLOCK_PC)) {
+		/*
+		 * Just check to see if the device is online.  If
+		 * it isn't, we refuse to process ordinary commands
+		 * (we will allow specials just in case someone needs
+		 * to send a command to an offline device without bringing
+		 * it back online)
+		 */
+		if(!sdev->online) {
+			printk(KERN_ERR "scsi%d (%d:%d): rejecting I/O to offline device\n",
+			       sdev->host->host_no, sdev->id, sdev->lun);
+			return BLKPREP_KILL;
+		}
 		/*
 		 * Now try and find a command block that we can use.
 		 */