netfilter: nf_reject: don't leak dst refcount for loopback packets

recent patches to add a WARN() when replacing skb dst entry found an
old bug:

WARNING: include/linux/skbuff.h:1165 skb_dst_check_unset include/linux/skbuff.h:1164 [inline]
WARNING: include/linux/skbuff.h:1165 skb_dst_set include/linux/skbuff.h:1210 [inline]
WARNING: include/linux/skbuff.h:1165 nf_reject_fill_skb_dst+0x2a4/0x330 net/ipv4/netfilter/nf_reject_ipv4.c:234
[..]
Call Trace:
nf_send_unreach+0x17b/0x6e0 net/ipv4/netfilter/nf_reject_ipv4.c:325
nft_reject_inet_eval+0x4bc/0x690 net/netfilter/nft_reject_inet.c:27
expr_call_ops_eval net/netfilter/nf_tables_core.c:237 [inline]
..

This is because blamed commit forgot about loopback packets.
Such packets already have a dst_entry attached, even at PRE_ROUTING stage.

Instead of checking hook just check if the skb already has a route
attached to it.

Fixes: f53b9b0bdc59 ("netfilter: introduce support for reject at prerouting stage")
Signed-off-by: Florian Westphal <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv4/netfilter/nf_reject_ipv4.c

···
 	if (!oth)
 		return;
 
-	if ((hook == NF_INET_PRE_ROUTING || hook == NF_INET_INGRESS) &&
-	    nf_reject_fill_skb_dst(oldskb) < 0)
+	if (!skb_dst(oldskb) && nf_reject_fill_skb_dst(oldskb) < 0)
 		return;
 
 	if (skb_rtable(oldskb)->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
···
 	if (iph->frag_off & htons(IP_OFFSET))
 		return;
 
-	if ((hook == NF_INET_PRE_ROUTING || hook == NF_INET_INGRESS) &&
-	    nf_reject_fill_skb_dst(skb_in) < 0)
+	if (!skb_dst(skb_in) && nf_reject_fill_skb_dst(skb_in) < 0)
 		return;
 
 	if (skb_csum_unnecessary(skb_in) ||

net/ipv6/netfilter/nf_reject_ipv6.c

···
 	fl6.fl6_sport = otcph->dest;
 	fl6.fl6_dport = otcph->source;
 
-	if (hook == NF_INET_PRE_ROUTING || hook == NF_INET_INGRESS) {
+	if (!skb_dst(oldskb)) {
 		nf_ip6_route(net, &dst, flowi6_to_flowi(&fl6), false);
 		if (!dst)
 			return;
···
 	if (hooknum == NF_INET_LOCAL_OUT && skb_in->dev == NULL)
 		skb_in->dev = net->loopback_dev;
 
-	if ((hooknum == NF_INET_PRE_ROUTING || hooknum == NF_INET_INGRESS) &&
-	    nf_reject6_fill_skb_dst(skb_in) < 0)
+	if (!skb_dst(skb_in) && nf_reject6_fill_skb_dst(skb_in) < 0)
 		return;
 
 	icmpv6_send(skb_in, ICMPV6_DEST_UNREACH, code, 0);