net: annotate data-races around sk->sk_max_pacing_rate

sk_getsockopt() runs locklessly. This means sk->sk_max_pacing_rate
can be read while other threads are changing its value.

Fixes: 62748f32 ("net: introduce SO_MAX_PACING_RATE")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>