ipv6: guard against possible NULL deref in __in6_dev_stats_get()

[ Upstream commit 507541c ]

dev_get_by_index_rcu() could return NULL if the original physical
device is unregistered.

Found by Sashiko.

Fixes: e1ae5c2 ("vrf: Increment Icmp6InMsgs on the original netdev")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Stephen Suryaputra <ssuryaextr@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260526145529.3587126-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: edumazet

include/net/addrconf.h

@@ -334,8 +334,11 @@ static inline struct inet6_dev *__in6_dev_get(const struct net_device *dev)
 static inline struct inet6_dev *__in6_dev_stats_get(const struct net_device *dev,
 						    const struct sk_buff *skb)
 {
-	if (netif_is_l3_master(dev))
+	if (netif_is_l3_master(dev)) {
 		dev = dev_get_by_index_rcu(dev_net(dev), inet6_iif(skb));
+		if (!dev)
+			return NULL;
+	}
 	return __in6_dev_get(dev);
 }