ACPI: custom_method: fix potential use-after-free issue

In the Linux kernel, the following vulnerability has been resolved:

ACPI: custom_method: fix potential use-after-free issue

In cm_write(), buf is always freed when reaching the end of the

function. If the requested count is less than table.length, the

allocated buffer will be freed but subsequent calls to cm_write() will

still try to access it.

Remove the unconditional kfree(buf) at the end of the function and

set the buf to NULL in the -EINVAL error path to match the rest of

function.