vfs: for usbfs, etc. internal vfsmounts ->mnt_sb->s_root == ->mnt_root

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index f3a257d..c9dc08d 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -160,7 +160,7 @@
 	 * have around.
 	 */
 	if (!parent)
-		parent = debugfs_mount->mnt_sb->s_root;
+		parent = debugfs_mount->mnt_root;
 
 	*dentry = NULL;
 	mutex_lock(&parent->d_inode->i_mutex);