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.