caif: Fix BUG() with network namespaces

The caif code will register its own pernet_operations, and then register
a netdevice_notifier. Each time the netdevice_notifier is triggered,
it'll do some stuff... including a lookup of its own pernet stuff with
net_generic().

If the net_generic() call ever returns NULL, the caif code will BUG().
That doesn't seem *so* unreasonable, I suppose — it does seem like it
should never happen.

However, it *does* happen. When we clone a network namespace,
setup_net() runs through all the pernet_operations one at a time. It
gets to loopback before it gets to caif. And loopback_net_init()
registers a netdevice... while caif hasn't been initialised. So the caif
netdevice notifier triggers, and immediately goes BUG().

We could imagine a complex and overengineered solution to this generic
class of problems, but this patch takes the simple approach. It just
makes caif_device_notify() *not* go looking for its pernet data
structures if the device it's being notified about isn't a caif device
in the first place.

Cc: stable@kernel.org
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Acked-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/caif/caif_dev.c

@@ -212,8 +212,7 @@ static int caif_device_notify(struct notifier_block *me, unsigned long what,
 	enum cfcnfg_phy_preference pref;
 	enum cfcnfg_phy_type phy_type;
 	struct cfcnfg *cfg;
-	struct caif_device_entry_list *caifdevs =
-	    caif_device_list(dev_net(dev));
+	struct caif_device_entry_list *caifdevs;
 
 	if (dev->type != ARPHRD_CAIF)
 		return 0;
@@ -222,6 +221,8 @@ static int caif_device_notify(struct notifier_block *me, unsigned long what,
 	if (cfg == NULL)
 		return 0;
 
+	caifdevs = caif_device_list(dev_net(dev));
+
 	switch (what) {
 	case NETDEV_REGISTER:
 		caifd = caif_device_alloc(dev);