cgroup: Use kvfree in pidlist_free()

The wrapper already calls the appropriate free
function, use it instead of spinning our own.

Signed-off-by: Bandan Das <bsd@redhat.com>
Acked-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --cc kernel/cgroup.c
Simple merge