cgroup: fix error handling regressions in proc_cgroup_show() and cgroup_release_agent()

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