inet: do not call sublist_rcv on empty list

syzbot triggered struct net NULL deref in NF_HOOK_LIST:
RIP: 0010:NF_HOOK_LIST include/linux/netfilter.h:331 [inline]
RIP: 0010:ip6_sublist_rcv+0x5c9/0x930 net/ipv6/ip6_input.c:292
ipv6_list_rcv+0x373/0x4b0 net/ipv6/ip6_input.c:328
__netif_receive_skb_list_ptype net/core/dev.c:5274 [inline]

Reason:
void ipv6_list_rcv(struct list_head *head, struct packet_type *pt,
struct net_device *orig_dev)
[..]
list_for_each_entry_safe(skb, next, head, list) {
/* iterates list */
skb = ip6_rcv_core(skb, dev, net);
/* ip6_rcv_core drops skb -> NULL is returned */
if (skb == NULL)
continue;
[..]
}
/* sublist is empty -> curr_net is NULL */
ip6_sublist_rcv(&sublist, curr_dev, curr_net);

Before the recent change NF_HOOK_LIST did a list iteration before
struct net deref, i.e. it was a no-op in the empty list case.

List iteration now happens after *net deref, causing crash.

Follow the same pattern as the ip(v6)_list_rcv loop and add a list_empty
test for the final sublist dispatch too.

Cc: Edward Cree <[e-mail]>
Reported-by: [e-mail]
Fixes: ca58fbe06c54 ("netfilter: add and use nf_hook_slow_list()")
Signed-off-by: Florian Westphal <[e-mail]>
Tested-by: Leon Romanovsky <[e-mail]>
Tested-by: Nikolay Aleksandrov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/ip_input.c

···
 		list_add_tail(&skb->list, &sublist);
 	}
 	/* dispatch final sublist */
-	ip_sublist_rcv(&sublist, curr_dev, curr_net);
+	if (!list_empty(&sublist))
+		ip_sublist_rcv(&sublist, curr_dev, curr_net);
 }

net/ipv6/ip6_input.c

···
 		list_add_tail(&skb->list, &sublist);
 	}
 	/* dispatch final sublist */
-	ip6_sublist_rcv(&sublist, curr_dev, curr_net);
+	if (!list_empty(&sublist))
+		ip6_sublist_rcv(&sublist, curr_dev, curr_net);
 }
 
 INDIRECT_CALLABLE_DECLARE(int udpv6_rcv(struct sk_buff *));