vrf: Stop corrupting skb->csum when capturing CHECKSUM_COMPLETE packets

The VRF device is an Ethernet device but it can have non-Ethernet ports
such as IP tunnels. Before the cited commit, capturing packets from such
ports on the VRF device resulted in these packets being detected as
malformed since they lack an Ethernet header.

The cited commit fixed it by pushing a dummy Ethernet header to such
packets before the capture and pulling it afterwards. In the case of
CHECKSUM_COMPLETE packets it also updated skb->csum with the checksum of
the dummy Ethernet header. This is wrong as skb->csum should not include
the checksum of the Ethernet header ("checksum of the _whole_ packet as
seen by netif_rx()").

This also means that L4 protocols receive a corrupted skb->csum and
potentially drop the packet, as is the case with UDP packets whose
checksum was completed by software.

Fix by removing the unnecessary call to skb_postpush_rcsum().

Fixes: 0489390882 ("vrf: add mac header for tunneled packets when sniffer is attached")
Reported-by: Stefano Sasso <stesasso@gmail.com>
Closes: https://lore.kernel.org/netdev/CALtE316UtL3x7LL6uxfXzx8rW6AbzYPeDOb478hqJCr_-dj=Wg@mail.gmail.com/
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Link: https://patch.msgid.link/20260922131239.2509494-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/vrf.c

@@ -1175,8 +1175,6 @@ static int vrf_prepare_mac_header(struct sk_buff *skb,
 	skb->protocol = eth->h_proto;
 	skb->pkt_type = PACKET_HOST;
 
-	skb_postpush_rcsum(skb, skb->data, ETH_HLEN);
-
 	skb_pull_inline(skb, ETH_HLEN);
 
 	return 0;