tcp: Fix a data-race around sysctl_tcp_notsent_lowat.

stable inclusion
from stable-v4.19.254
commit 0f75343584ee474303e17efe0610bdd170af1d13
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ


CVE: NA

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[ Upstream commit 55be873695ed8912eb77ff46d1d1cadf028bd0f3 ]

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

Fixes: c9bee3b7 ("tcp: TCP_NOTSENT_LOWAT socket option")
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>