tcp: Fix a data-race around sysctl_tcp_min_rtt_wlen.

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

Fixes: f6722583

 ("tcp: track min RTT using windowed min-filter")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>