clean RTM_HOST flag for ipv6 route

[gaoxingwang]

Keep action same with ip command

Before the patch: $ip -6 route add 2409:8080:5a0a:60c7::7/128 via 2409:8080:5a0a:60c7::7 dev eth2 $ip -6 route add 2409:8080:5a0a:60c7::8/128 via 2409:8080:5a0a:60c7::7 dev eth2 RTNETLINK answers: No route to host $ route -A inet6 add 2409:8080:5a0a:60c7::8/128 gw 2409:8080:5a0a:60c7::7 dev eth2 $ After the patch: $route -A inet6 add 2409:8080:5a0a:60c7::8/128 gw 2409:8080:5a0a:60c7::7 dev eth2 SIOCADDRT: No route to host

fixes #27