cgroup: remove redundant check in cgroup_ino()

After we implemented default unified hierarchy, cgrp->kn can never
be NULL.

Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 77a1d37..818a81f 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -532,13 +532,10 @@
 	return !list_empty(&cgrp->cset_links);
 }
 
-/* returns ino associated with a cgroup, 0 indicates unmounted root */
+/* returns ino associated with a cgroup */
 static inline ino_t cgroup_ino(struct cgroup *cgrp)
 {
-	if (cgrp->kn)
-		return cgrp->kn->ino;
-	else
-		return 0;
+	return cgrp->kn->ino;
 }
 
 /* cft/css accessors for cftype->write() operation */