tcp: Fix a data-race around sysctl_tcp_comp_sack_slack_ns.

While reading sysctl_tcp_comp_sack_slack_ns, it can be changed
concurrently.  Thus, we need to add READ_ONCE() to its reader.

Fixes: a70437cc ("tcp: add hrtimer slack to sack compression")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -5547,7 +5547,7 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
 		      rtt * (NSEC_PER_USEC >> 3)/20);
 	sock_hold(sk);
 	hrtimer_start_range_ns(&tp->compressed_ack_timer, ns_to_ktime(delay),
-			       sock_net(sk)->ipv4.sysctl_tcp_comp_sack_slack_ns,
+			       READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_slack_ns),
 			       HRTIMER_MODE_REL_PINNED_SOFT);
 }