iproute: Set ip/ip6 lwtunnel flags

iproute: Set ip/ip6 lwtunnel flags

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

User can set flags with key, csum, seq
ip r a 10.0.0.1 encap ip dst 192.168.152.171 id 1000 key csum dev gretap

Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>