tcp: annotate tp->write_seq lockless reads

There are few places where we fetch tp->write_seq while
this field can change from IRQ or other cpu.

We need to add READ_ONCE() annotations, and also make
sure write sides use corresponding WRITE_ONCE() to avoid
store-tearing.

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

include/net/tcp.h

@@ -1917,7 +1917,7 @@ static inline u32 tcp_notsent_lowat(const struct tcp_sock *tp)
 static inline bool tcp_stream_memory_free(const struct sock *sk, int wake)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
-	u32 notsent_bytes = tp->write_seq - tp->snd_nxt;
+	u32 notsent_bytes = READ_ONCE(tp->write_seq) - tp->snd_nxt;
 
 	return (notsent_bytes << wake) < tcp_notsent_lowat(tp);
 }