commit a1eb03f5 upstream.

The reason we defer kfree until release function is because it's a
general rule for kobjects: kfree of the reference counter itself is only
legal in the release function.

Previous patch didn't make this clear, document this in code.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
[lizf: Backported to 3.4: adjust filename]
Signed-off-by: Zefan Li <lizefan@huawei.com>