ipvlan: Ignore PACKET_LOOPBACK in handle_mode_l2()

Packets with pkt_type == PACKET_LOOPBACK are captured by
handle_frame() function, but they don't have L2 header.
We should not process them in handle_mode_l2().

This doesn't affect old L2 functionality, since handling
was anyway incorrect.

Handle them the same way as in br_handle_frame():
just pass the skb.

To observe invalid behaviour, just start "ping -b" on bcast address
of port-interface.

Fixes: 2ad7bf3 ("ipvlan: Initial check-in of the IPVLAN driver.")
Signed-off-by: Dmitry Skorodumov <skorodumov.dmitry@huawei.com>
Link: https://patch.msgid.link/20251202103906.4087675-1-skorodumov.dmitry@huawei.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Dmitry Skorodumov

drivers/net/ipvlan/ipvlan_core.c

@@ -737,6 +737,9 @@ static rx_handler_result_t ipvlan_handle_mode_l2(struct sk_buff **pskb,
 	struct ethhdr *eth = eth_hdr(skb);
 	rx_handler_result_t ret = RX_HANDLER_PASS;
 
+	if (unlikely(skb->pkt_type == PACKET_LOOPBACK))
+		return RX_HANDLER_PASS;
+
 	if (is_multicast_ether_addr(eth->h_dest)) {
 		if (ipvlan_external_frame(skb, port)) {
 			struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);