tcp: Fix a data-race around sysctl_tcp_stdurg.

stable inclusion
from stable-v4.19.254
commit e570964c9f36fa386db95cc776d2e1dc16e69496
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ


CVE: NA

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[ Upstream commit 4e08ed41cb1194009fc1a916a59ce3ed4afd77cd ]

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

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
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>