tcp: Fix data-races around sysctl_tcp_dsack.

stable inclusion
from stable-v4.19.255
commit 62acabe02a2b1a65be563e1a8825f80dcb059aed
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ


CVE: NA

--------------------------------

commit 58ebb1c8b35a8ef38cd6927431e0fa7b173a632d upstream.

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

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>