devres: WARN() and return, don't crash on device_del() of uninitialized device

I just debugged an obscure crash caused by a device_del() of a all NULL'd
out struct device (in usb-serial) and found that a patch like this one would
have saved me time (in addition to improved chances of a bug report from
users hitting similar driver bugs).

[[e-mail]: cleanup]
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>
Cc: Kay Sievers <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/base/devres.c

···
 {
 	unsigned long flags;
 
+	/* Looks like an uninitialized device structure */
+	if (WARN_ON(dev->devres_head.next == NULL))
+		return -ENODEV;
 	spin_lock_irqsave(&dev->devres_lock, flags);
 	return release_nodes(dev, dev->devres_head.next, &dev->devres_head,
 			     flags);