ip: Fix a data-race around sysctl_fwmark_reflect.

stable inclusion
from stable-v4.19.254
commit 9096edcf4854289f92252e086cf6e498c7f8c21d
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ


CVE: NA

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

[ Upstream commit 85d0b4dbd74b95cc492b1f4e34497d3f894f5d9a ]

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

Fixes: e110861f ("net: add a sysctl to reflect the fwmark on replies")
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>