tcp: Fix data-races around sysctl_tcp_dsack.

stable inclusion
from stable-v5.10.135
commit f10a5f905a97cb31e0d8a151c5af5fd2e4494f91
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5ZWFM

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f10a5f905a97cb31e0d8a151c5af5fd2e4494f91



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commit 58ebb1c8 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: Zheng Zengkai <zhengzengkai@huawei.com>
Reviewed-by: Wei Li <liwei391@huawei.com>