openvswitch: Only set TUNNEL_VXLAN_OPT if VXLAN-GBP metadata is set

This avoids setting TUNNEL_VXLAN_OPT for VXLAN frames which don't
have any GBP metadata set. It is not invalid to set it but unnecessary.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>