tcp: Fix a data-race around sysctl_tcp_challenge_ack_limit.

stable inclusion
from stable-v4.19.255
commit 3bf8415f605526a7eea9f3f7d7e30d8867b2cd1e
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ


CVE: NA

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

commit db3815a2 upstream.

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

Fixes: 282f23c6 ("tcp: implement RFC 5961 3.2")
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>