sock: correctly test SOCK_TIMESTAMP in sock_recv_ts_and_drops()

It seems the code does not match the intent.

This broke packetdrill, and probably other programs.

Fixes: 6c7c98b ("sock: avoid dirtying sk_stamp, if possible")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -2250,7 +2250,7 @@ static inline void sock_recv_ts_and_drops(struct msghdr *msg, struct sock *sk,
 
 	if (sk->sk_flags & FLAGS_TS_OR_DROPS || sk->sk_tsflags & TSFLAGS_ANY)
 		__sock_recv_ts_and_drops(msg, sk, skb);
-	else if (unlikely(sk->sk_flags & SOCK_TIMESTAMP))
+	else if (unlikely(sock_flag(sk, SOCK_TIMESTAMP)))
 		sk->sk_stamp = skb->tstamp;
 	else if (unlikely(sk->sk_stamp == SK_DEFAULT_STAMP))
 		sk->sk_stamp = 0;