tcp: Fix a data-race around sysctl_tcp_comp_sack_delay_ns.

stable inclusion
from stable-v4.19.255
commit 32917d384b89836e593aa8ef795508257fd7d102
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ


CVE: NA

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

[ Upstream commit 4866b2b0f7672b6d760c4b8ece6fb56f965dcc8a ]

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

Fixes: 6d82aa24 ("tcp: add tcp_comp_sack_delay_ns sysctl")
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>