net: annotate data-races around sk->sk_tsflags

sk->sk_tsflags can be read locklessly, add corresponding annotations.

Fixes: b9f40e21

 ("net-timestamp: move timestamp flags out of sk_flags")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>