xen/blkfront: Fix crash if backend doesn't follow the right states.

We have split the setting up of all the resources in two steps:
1) talk_to_blkback  - which figures out the num_ring_pages (from
   the default value of zero), sets up shadow and so
2) blkfront_connect - does the real part of filling out the
   internal structures.

The problem is if we bypass the 1) step and go straight to 2)
and call blkfront_setup_indirect where we use the macro
BLK_RING_SIZE - which returns an negative value (because
sz is zero  - since num_ring_pages is zero - since it has never
been set).

We can fix this by making sure that we always have called
talk_to_blkback before going to blkfront_connect.

Or we could set in blkfront_probe info->nr_ring_pages = 1
to have a default value. But that looks odd - as we haven't
actually negotiated any ring size.

This patch changes XenbusStateConnected state to detect if
we haven't done the initial handshake - and if so continue
on as if were in XenbusStateInitWait state.

We also roll the error recovery (freeing the structure) into
talk_to_blkback error path - which is safe since that function
is only called from blkback_changed.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkfront.c

@@ -1867,6 +1867,9 @@ static int talk_to_blkback(struct xenbus_device *dev,
  destroy_blkring:
 	blkif_free(info, 0);
 
+	kfree(info);
+	dev_set_drvdata(&dev->dev, NULL);
+
 	return err;
 }
 
@@ -2453,11 +2456,8 @@ static void blkback_changed(struct xenbus_device *dev,
 	case XenbusStateInitWait:
 		if (dev->state != XenbusStateInitialising)
 			break;
-		if (talk_to_blkback(dev, info)) {
-			kfree(info);
-			dev_set_drvdata(&dev->dev, NULL);
+		if (talk_to_blkback(dev, info))
 			break;
-		}
 	case XenbusStateInitialising:
 	case XenbusStateInitialised:
 	case XenbusStateReconfiguring:
@@ -2466,6 +2466,10 @@ static void blkback_changed(struct xenbus_device *dev,
 		break;
 
 	case XenbusStateConnected:
+		if (dev->state != XenbusStateInitialised) {
+			if (talk_to_blkback(dev, info))
+				break;
+		}
 		blkfront_connect(info);
 		break;