ipv6: fix a possible double free

When calling snmp6_alloc_dev fails, the snmp6 relevant memory
are freed by snmp6_alloc_dev. Calling in6_dev_finish_destroy
will free these memory twice.

Double free will lead that undefined behavior occurs.
Signed-off-by: NRoy Li <rongqing.li@windriver.com>
Acked-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -374,8 +374,8 @@ static struct inet6_dev * ipv6_add_dev(struct net_device *dev)
 			"%s(): cannot allocate memory for statistics; dev=%s.\n",
 			__func__, dev->name));
 		neigh_parms_release(&nd_tbl, ndev->nd_parms);
-		ndev->dead = 1;
-		in6_dev_finish_destroy(ndev);
+		dev_put(dev);
+		kfree(ndev);
 		return NULL;
 	}