vfs: document ->d_real()

Add missing documentation for the d_op->d_real() method and d_real()
helper.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 75eea7c..898d3ce 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -20,6 +20,8 @@
 	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
 	struct vfsmount *(*d_automount)(struct path *path);
 	int (*d_manage)(struct dentry *, bool);
+	struct dentry *(*d_real)(struct dentry *, const struct inode *,
+				 unsigned int);
 
 locking rules:
 		rename_lock	->d_lock	may block	rcu-walk
@@ -34,6 +36,7 @@
 d_dname:	no		no		no		no
 d_automount:	no		no		yes		no
 d_manage:	no		no		yes (ref-walk)	maybe
+d_real		no		no		yes 		no
 
 --------------------------- inode_operations --------------------------- 
 prototypes: