net: ipv4: fix stat increase when udp early demux drops the packet

udp_v4_early_demux now returns drop reasons as it either returns 0 or
ip_mc_validate_source, which returns itself a drop reason. However its
use was not converted in ip_rcv_finish_core and the drop reason is
ignored, leading to potentially skipping increasing LINUX_MIB_IPRPFILTER
if the drop reason is SKB_DROP_REASON_IP_RPFILTER.

This is a fix and we're not converting udp_v4_early_demux to explicitly
return a drop reason to ease backports; this can be done as a follow-up.

Fixes: d46f827 ("net: ip: make ip_mc_validate_source() return drop reason")
Cc: Menglong Dong <menglong8.dong@gmail.com>
Reported-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250701074935.144134-1-atenart@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: atenart

net/ipv4/ip_input.c

@@ -319,8 +319,8 @@ static int ip_rcv_finish_core(struct net *net,
 			      const struct sk_buff *hint)
 {
 	const struct iphdr *iph = ip_hdr(skb);
-	int err, drop_reason;
 	struct rtable *rt;
+	int drop_reason;
 
 	if (ip_can_use_hint(skb, iph, hint)) {
 		drop_reason = ip_route_use_hint(skb, iph->daddr, iph->saddr,
@@ -345,9 +345,10 @@ static int ip_rcv_finish_core(struct net *net,
 			break;
 		case IPPROTO_UDP:
 			if (READ_ONCE(net->ipv4.sysctl_udp_early_demux)) {
-				err = udp_v4_early_demux(skb);
-				if (unlikely(err))
+				drop_reason = udp_v4_early_demux(skb);
+				if (unlikely(drop_reason))
 					goto drop_error;
+				drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
 
 				/* must reload iph, skb->head might have changed */
 				iph = ip_hdr(skb);