cpuset: fix error handling regression in proc_cpuset_show()

4c737b41 ("cgroup: make cgroup_path() and friends behave in the
style of strlcpy()") botched the conversion of proc_cpuset_show() and
broke its error handling.  It made the function return 0 on failures
and fail to handle error returns from cgroup_path_ns().  Fix it.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cpuset.c

@@ -2698,12 +2698,13 @@ int proc_cpuset_show(struct seq_file *m, struct pid_namespace *ns,
 	if (!buf)
 		goto out;
 
-	retval = -ENAMETOOLONG;
 	css = task_get_css(tsk, cpuset_cgrp_id);
 	retval = cgroup_path_ns(css->cgroup, buf, PATH_MAX,
 				current->nsproxy->cgroup_ns);
 	css_put(css);
 	if (retval >= PATH_MAX)
+		retval = -ENAMETOOLONG;
+	if (retval < 0)
 		goto out_free;
 	seq_puts(m, buf);
 	seq_putc(m, '\n');
@@ -2711,7 +2712,7 @@ int proc_cpuset_show(struct seq_file *m, struct pid_namespace *ns,
 out_free:
 	kfree(buf);
 out:
-	return 0;
+	return retval;
 }
 #endif /* CONFIG_PROC_PID_CPUSET */