tcp: Fix data-races around sysctl_tcp_no_ssthresh_metrics_save.

tcp: Fix data-races around sysctl_tcp_no_ssthresh_metrics_save.

commit 7b8512603083cf7666210748820e9b22a8977b2a upstream.

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

Fixes: 1cb407385d84 ("net-tcp: Disable TCP ssthresh metrics cache by default")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>