iptunnel: Set tun_flags in the iptunnel_metadata_reply from src

ip l add tun type gretap external
ip r a 10.0.0.2 encap ip id 1000 dst 172.168.0.2 key dev tun
ip a a 10.0.0.1/24 dev tun

The peer arp request to 10.0.0.1 with tunnel_id, but the arp reply
only set the tun_id but not the tun_flags with TUNNEL_KEY. The arp
reply packet don't contain tun_id field.

Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>