tcp: annotate tp->copied_seq lockless reads

There are few places where we fetch tp->copied_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.

Note that tcp_inq_hint() was already using READ_ONCE(tp->copied_seq)

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

net/ipv6/tcp_ipv6.c

@@ -1896,7 +1896,7 @@ static void get_tcp6_sock(struct seq_file *seq, struct sock *sp, int i)
 		 * we might find a transient negative value.
 		 */
 		rx_queue = max_t(int, READ_ONCE(tp->rcv_nxt) -
-				      tp->copied_seq, 0);
+				      READ_ONCE(tp->copied_seq), 0);
 
 	seq_printf(seq,
 		   "%4d: %08X%08X%08X%08X:%04X %08X%08X%08X%08X:%04X "