vxlan: set mac_header correctly in GPE mode

For VXLAN-GPE, the interface is ARPHRD_NONE, thus we need to reset
mac_header after pulling the outer header.

v2: Put the code to the existing conditional block as suggested by
    Shmulik Ladkani.

Fixes: e1e5314de08b ("vxlan: implement GPE")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Reviewed-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/vxlan.c b/drivers/net/vxlan.c
index 2f29d20..25ab6bf 100644
--- a/drivers/net/vxlan.c
+++ b/drivers/net/vxlan.c
@@ -1381,6 +1381,7 @@
 		if (!vxlan_set_mac(vxlan, vs, skb))
 			goto drop;
 	} else {
+		skb_reset_mac_header(skb);
 		skb->dev = vxlan->dev;
 		skb->pkt_type = PACKET_HOST;
 	}