tcp: use dst_dev_rcu() in tcp_fastopen_active_disable_ofo_check()

Use RCU to avoid a pair of atomic operations and a potential
UAF on dst_dev()->flags.

Signed-off-by: Eric Dumazet <[e-mail]>
Reviewed-by: David Ahern <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv4/tcp_fastopen.c

···
 		}
 	} else if (tp->syn_fastopen_ch &&
 		   atomic_read(&sock_net(sk)->ipv4.tfo_active_disable_times)) {
-		dst = sk_dst_get(sk);
-		dev = dst ? dst_dev(dst) : NULL;
+		rcu_read_lock();
+		dst = __sk_dst_get(sk);
+		dev = dst ? dst_dev_rcu(dst) : NULL;
 		if (!(dev && (dev->flags & IFF_LOOPBACK)))
 			atomic_set(&sock_net(sk)->ipv4.tfo_active_disable_times, 0);
-		dst_release(dst);
+		rcu_read_unlock();
 	}
 }