u32: add const to rtnl_u32_add_key_in*_addr()

Cc: Thomas Haller <thaller@redhat.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Thomas Haller <thaller@redhat.com>