dccp: call inet_add_protocol after register_pernet_subsys in dccp_v6_init

Patch "call inet_add_protocol after register_pernet_subsys in dccp_v4_init"
fixed a null pointer dereference issue for dccp_ipv4 module.

The same fix is needed for dccp_ipv6 module.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>