vxlan: Only set has-GBP bit in header if any other bits would be set

This allows for a VXLAN-GBP socket to talk to a Linux VXLAN socket by
not setting any of the bits.

Signed-off-by: Thomas Graf <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/vxlan.c

···
 {
 	struct vxlanhdr_gbp *gbp;
 
+	if (!md->gbp)
+		return;
+
 	gbp = (struct vxlanhdr_gbp *)vxh;
 	vxh->vx_flags |= htonl(VXLAN_HF_GBP);