The NLM_F_DUMP_PROPER_HDR netlink flag was replaced by a setsockopt.
Update the comment in rtnl_stats_dump.

Fixes: 841891ec ("rtnetlink: Update rtnl_stats_dump for strict data checking")
Reported-by: Christian Brauner <christian@brauner.io>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>