4c737b41 ("cgroup: make cgroup_path() and friends behave in the
style of strlcpy()") broke error handling in proc_cgroup_show() and
cgroup_release_agent() by not handling negative return values from
cgroup_path_ns_locked().  Fix it.
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NTejun Heo <tj@kernel.org>