cgroup: fix and restructure error handling in copy_cgroup_ns()

copy_cgroup_ns()'s error handling was broken and the attempt to fix it
d2202557 ("cgroup: fix alloc_cgroup_ns() error handling in
copy_cgroup_ns()") was broken too in that it ended up trying an
ERR_PTR() value.

There's only one place where copy_cgroup_ns() needs to perform cleanup
after failure.  Simplify and fix the error handling by removing the
goto's.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>