ipv6: annotate data-races in ndisc_router_discovery()

Annotate reads from in6_dev->cnf.XXX fields, as they could
change concurrently.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>