net: Fix oops in dev_ifsioc()

A command like this: "brctl addif br1 eth1" issued as a user gave me
an oops when bridge module wasn't loaded. It's caused by using a dev
pointer before checking for NULL.
Signed-off-by: NJarek Poplawski <jarkao2@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -3745,11 +3745,13 @@ static int dev_ifsioc(struct net *net, struct ifreq *ifr, unsigned int cmd)
 {
 	int err;
 	struct net_device *dev = __dev_get_by_name(net, ifr->ifr_name);
-	const struct net_device_ops *ops = dev->netdev_ops;
+	const struct net_device_ops *ops;
 
 	if (!dev)
 		return -ENODEV;
 
+	ops = dev->netdev_ops;
+
 	switch (cmd) {
 		case SIOCSIFFLAGS:	/* Set interface flags */
 			return dev_change_flags(dev, ifr->ifr_flags);