net: vlan: fix underflow for the real_dev refcnt

Inject error before dev_hold(real_dev) in register_vlan_dev(),
and execute the following testcase:

ip link add dev dummy1 type dummy
ip link add name dummy1.100 link dummy1 type vlan id 100
ip link del dev dummy1

When the dummy netdevice is removed, we will get a WARNING as following:

=======================================================================
refcount_t: decrement hit 0; leaking memory.
WARNING: CPU: 2 PID: 0 at lib/refcount.c:31 refcount_warn_saturate+0xbf/0x1e0

and an endless loop of:

=======================================================================
unregister_netdevice: waiting for dummy1 to become free. Usage count = -1073741824

That is because dev_put(real_dev) in vlan_dev_free() be called without
dev_hold(real_dev) in register_vlan_dev(). It makes the refcnt of real_dev
underflow.

Move the dev_hold(real_dev) to vlan_dev_init() which is the call-back of
ndo_init(). That makes dev_hold() and dev_put() for vlan's real_dev
symmetrical.

Fixes: 563bcbae3ba2 ("net: vlan: fix a UAF in vlan_dev_real_dev()")
Reported-by: Petr Machata <[e-mail]>
Suggested-by: Jakub Kicinski <[e-mail]>
Signed-off-by: Ziyang Xuan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/8021q/vlan.c

···
 	if (err)
 		goto out_unregister_netdev;
 
-	/* Account for reference in struct vlan_dev_priv */
-	dev_hold(real_dev);
-
 	vlan_stacked_transfer_operstate(real_dev, dev, vlan);
 	linkwatch_fire_event(dev); /* _MUST_ call rfc2863_policy() */
 

net/8021q/vlan_dev.c

···
 	if (!vlan->vlan_pcpu_stats)
 		return -ENOMEM;
 
+	/* Get vlan's reference to real_dev */
+	dev_hold(real_dev);
+
 	return 0;
 }