cgroup: fix alloc_cgroup_ns() error handling in copy_cgroup_ns()

alloc_cgroup_ns() returns an ERR_PTR value on error but
copy_cgroup_ns() was checking for NULL for error.  Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>