netrom: Fix data-races around sysctl_net_busy_read

stable inclusion
from stable-v5.10.213
commit bbf950a6e96a91cf8cf0c71117b94ed3fafc9dd3
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9Q94D
CVE: CVE-2024-27419

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bbf950a6e96a91cf8cf0c71117b94ed3fafc9dd3



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[ Upstream commit d380ce70058a4ccddc3e5f5c2063165dc07672c6 ]

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: Hongbo Li <lihongbo22@huawei.com>