acct: block access to kernel internal filesystems

There's no point in allowing anything kernel internal nor procfs or
sysfs.

Link: https://lore.kernel.org/r/20250127091811.3183623-1-quzicheng@huawei.com
Link: https://lore.kernel.org/r/20250211-work-acct-v1-2-1c16aecab8b3@kernel.org


Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reported-by: Zicheng Qu <quzicheng@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>