netrom: Fix data-races around sysctl_netrom_network_ttl_initialiser

stable inclusion
from stable-v4.19.310
commit acc653e8a3aaab1b7103f98645f2cce7be89e3d3
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9Q91I
CVE: CVE-2024-27428

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=acc653e8a3aaab1b7103f98645f2cce7be89e3d3



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[ Upstream commit 119cae5ea3f9e35cdada8e572cc067f072fa825a ]

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: Zhao Wenhui <zhaowenhui8@huawei.com>