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: e1e5314d ("vxlan: implement GPE")
Signed-off-by: NJiri Benc <jbenc@redhat.com>
Reviewed-by: NShmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -1381,6 +1381,7 @@ static int vxlan_rcv(struct sock *sk, struct sk_buff *skb)
 		if (!vxlan_set_mac(vxlan, vs, skb))
 			goto drop;
 	} else {
+		skb_reset_mac_header(skb);
 		skb->dev = vxlan->dev;
 		skb->pkt_type = PACKET_HOST;
 	}