tcp: Fix a data-race around sysctl_tcp_min_rtt_wlen.

stable inclusion
from stable-v4.19.255
commit 3a73df9db9297ada3e58776810c8e52332f3f1a0
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ


CVE: NA

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[ Upstream commit 1330ffacd05fc9ac4159d19286ce119e22450ed2 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>