net: fix segmentation after TCP/UDP fraglist GRO

JIRA: https://issues.redhat.com/browse/RHEL-115580

Upstream commit:
commit 9f735b6
Author: Felix Fietkau <nbd@nbd.name>
Date:   Sat Jul 5 17:06:21 2025 +0200

    net: fix segmentation after TCP/UDP fraglist GRO

    Since "net: gro: use cb instead of skb->network_header", the skb network
    header is no longer set in the GRO path.
    This breaks fraglist segmentation, which relies on ip_hdr()/tcp_hdr()
    to check for address/port changes.
    Fix this regression by selectively setting the network header for merged
    segment skbs.

    Fixes: 186b1ea ("net: gro: use cb instead of skb->network_header")
    Signed-off-by: Felix Fietkau <nbd@nbd.name>
    Reviewed-by: Willem de Bruijn <willemb@google.com>
    Link: https://patch.msgid.link/20250705150622.10699-1-nbd@nbd.name
    Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Paolo Abeni

net/ipv4/tcp_offload.c

@@ -355,6 +355,7 @@ struct sk_buff *tcp_gro_receive(struct list_head *head, struct sk_buff *skb,
 		flush |= skb->ip_summed != p->ip_summed;
 		flush |= skb->csum_level != p->csum_level;
 		flush |= NAPI_GRO_CB(p)->count >= 64;
+		skb_set_network_header(skb, skb_gro_receive_network_offset(skb));
 
 		if (flush || skb_gro_receive_list(p, skb))
 			mss = 1;

net/ipv4/udp_offload.c

@@ -604,6 +604,7 @@ static struct sk_buff *udp_gro_receive_segment(struct list_head *head,
 					NAPI_GRO_CB(skb)->flush = 1;
 					return NULL;
 				}
+				skb_set_network_header(skb, skb_gro_receive_network_offset(skb));
 				ret = skb_gro_receive_list(p, skb);
 			} else {
 				skb_gro_postpull_rcsum(skb, uh,