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/security/inode.c b/security/inode.c
index c4df2fb..a67004f 100644
--- a/security/inode.c
+++ b/security/inode.c
@@ -159,7 +159,7 @@
 	 * have around.
 	 */
 	if (!parent)
-		parent = mount->mnt_sb->s_root;
+		parent = mount->mnt_root;
 
 	mutex_lock(&parent->d_inode->i_mutex);
 	*dentry = lookup_one_len(name, parent, strlen(name));