nvmem: core: limit cell sysfs permissions to main attribute ones

In the Linux kernel, the following vulnerability has been resolved: nvmem: core: limit cell sysfs permissions to main attribute ones The cell sysfs attribute should not provide more access to the nvmem data than the main attribute itself. For example if nvme_config::root_only was set, the cell attribute would still provide read access to everybody. Mask out permissions not available on the main attribute.