fs: rcu-walk aware d_revalidate method

Require filesystems be aware of .d_revalidate being called in rcu-walk
mode (nd->flags & LOOKUP_RCU). For now do a simple push down, returning
-ECHILD from all implementations.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

Documentation/filesystems/Locking

@@ -9,7 +9,7 @@ be able to use diff(1).
 
 --------------------------- dentry_operations --------------------------
 prototypes:
-	int (*d_revalidate)(struct dentry *, int);
+	int (*d_revalidate)(struct dentry *, struct nameidata *);
 	int (*d_hash)(const struct dentry *, const struct inode *,
 			struct qstr *);
 	int (*d_compare)(const struct dentry *, const struct inode *,
@@ -21,14 +21,14 @@ prototypes:
 	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
 
 locking rules:
-		rename_lock	->d_lock	may block
-d_revalidate:	no		no		yes
-d_hash		no		no		no
-d_compare:	yes		no		no
-d_delete:	no		yes		no
-d_release:	no		no		yes
-d_iput:		no		no		yes
-d_dname:	no		no		no
+		rename_lock	->d_lock	may block	rcu-walk
+d_revalidate:	no		no		yes (ref-walk)	maybe
+d_hash		no		no		no		maybe
+d_compare:	yes		no		no		maybe
+d_delete:	no		yes		no		no
+d_release:	no		no		yes		no
+d_iput:		no		no		yes		no
+d_dname:	no		no		no		no
 
 --------------------------- inode_operations --------------------------- 
 prototypes: