iptunnel: make TUNNEL_FLAGS available in uapi

ip l add dev tun type gretap external
ip r a 10.0.0.1 encap ip dst 192.168.152.171 id 1000 dev gretap

For gretap Key example when the command set the id but don't set the
TUNNEL_KEY flags. There is no key field in the send packet

In the lwtunnel situation, some TUNNEL_FLAGS should can be set by
userspace

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