netrom: Fix a data-race around sysctl_netrom_transport_timeout

stable inclusion
from stable-v4.19.310
commit 500936692ccca8617a955652d1929f079b17a201
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9Q95M
CVE: CVE-2024-27427

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=500936692ccca8617a955652d1929f079b17a201



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

[ Upstream commit 60a7a152abd494ed4f69098cf0f322e6bb140612 ]

We need to protect the reader reading the sysctl value because the
value can be changed concurrently.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Hui Tang <tanghui20@huawei.com>