ipv6: annotate data-races in rt6_probe()

Use READ_ONCE() while reading idev->cnf.rtr_probe_interval
while its value could be changed.

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