tcp: Fix a data-race around sysctl_tcp_mtu_probe_floor.

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

Fixes: c04b79b6 ("tcp: add new tcp_mtu_probe_floor sysctl")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_timer.c

@@ -172,7 +172,7 @@ static void tcp_mtu_probing(struct inet_connection_sock *icsk, struct sock *sk)
 	} else {
 		mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
 		mss = min(READ_ONCE(net->ipv4.sysctl_tcp_base_mss), mss);
-		mss = max(mss, net->ipv4.sysctl_tcp_mtu_probe_floor);
+		mss = max(mss, READ_ONCE(net->ipv4.sysctl_tcp_mtu_probe_floor));
 		mss = max(mss, READ_ONCE(net->ipv4.sysctl_tcp_min_snd_mss));
 		icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
 	}