kernfs: fix kernfs_node_from_dentry()

Currently kernfs_node_from_dentry() returns NULL for root dentry,
because root_dentry->d_op == NULL.

Due to this bug cgroupstats_build() returns -EINVAL for root cgroup.

  # mount -t cgroup -o cpuacct /cgroup
  # Documentation/accounting/getdelays -C /cgroup
  fatal reply error,  errno -22

With this fix:

  # Documentation/accounting/getdelays -C /cgroup
  sleeping 305, blocked 0, running 1, stopped 0, uninterruptible 1

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/dir.c

@@ -500,7 +500,7 @@ const struct dentry_operations kernfs_dops = {
  */
 struct kernfs_node *kernfs_node_from_dentry(struct dentry *dentry)
 {
-	if (dentry->d_op == &kernfs_dops)
+	if (dentry->d_sb->s_op == &kernfs_sops)
 		return dentry->d_fsdata;
 	return NULL;
 }

fs/kernfs/kernfs-internal.h

@@ -65,6 +65,7 @@ struct kernfs_super_info {
 };
 #define kernfs_info(SB) ((struct kernfs_super_info *)(SB->s_fs_info))
 
+extern const struct super_operations kernfs_sops;
 extern struct kmem_cache *kernfs_node_cache;
 
 /*

fs/kernfs/mount.c

@@ -39,7 +39,7 @@ static int kernfs_sop_show_options(struct seq_file *sf, struct dentry *dentry)
 	return 0;
 }
 
-static const struct super_operations kernfs_sops = {
+const struct super_operations kernfs_sops = {
 	.statfs		= simple_statfs,
 	.drop_inode	= generic_delete_inode,
 	.evict_inode	= kernfs_evict_inode,