ieee802154: Fix oops during ieee802154_sock_ioctl

Trying to run izlisten (from lowpan-tools tests) on a device that does not
exists I got the oops below. The problem is that we are using get_dev_by_name
without checking if we really get a device back. We don't in this case and
writing to dev->type generates this oops.

[Oops code removed by Dmitry Eremin-Solenikov]

If possible this patch should be applied to the current -rc fixes branch.
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ieee802154/af_ieee802154.c

@@ -151,6 +151,9 @@ static int ieee802154_dev_ioctl(struct sock *sk, struct ifreq __user *arg,
 	dev_load(sock_net(sk), ifr.ifr_name);
 	dev = dev_get_by_name(sock_net(sk), ifr.ifr_name);
 
+	if (!dev)
+		return -ENODEV;
+
 	if (dev->type == ARPHRD_IEEE802154 && dev->netdev_ops->ndo_do_ioctl)
 		ret = dev->netdev_ops->ndo_do_ioctl(dev, &ifr, cmd);