macvtap: fix network header pointer for VLAN tagged pkts

Network header is set with offset ETH_HLEN but it is not true for VLAN
(multiple-)tagged and results in checksum issues in lower devices.

v2: leave skb->protocol untouched (thx Vlad), comment added
v3: moved after skb_probe_transport_header() call (thx Toshiaki)
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macvtap.c

@@ -719,6 +719,7 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
 	struct virtio_net_hdr vnet_hdr = { 0 };
 	int vnet_hdr_len = 0;
 	int copylen = 0;
+	int depth;
 	bool zerocopy = false;
 	size_t linear;
 	ssize_t n;
@@ -804,6 +805,12 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
 
 	skb_probe_transport_header(skb, ETH_HLEN);
 
+	/* Move network header to the right position for VLAN tagged packets */
+	if ((skb->protocol == htons(ETH_P_8021Q) ||
+	     skb->protocol == htons(ETH_P_8021AD)) &&
+	    __vlan_get_protocol(skb, skb->protocol, &depth) != 0)
+		skb_set_network_header(skb, depth);
+
 	rcu_read_lock();
 	vlan = rcu_dereference(q->vlan);
 	/* copy skb_ubuf_info for callback when skb has no error */