net: annotate data-races around sk->sk_bind_phc

sk->sk_bind_phc is read locklessly. Add corresponding annotations.

Fixes: d463126e ("net: sock: extend SO_TIMESTAMPING for PHC binding")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>