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>

fs/cifs/dir.c

@@ -656,6 +656,9 @@ lookup_out:
 static int
 cifs_d_revalidate(struct dentry *direntry, struct nameidata *nd)
 {
+	if (nd->flags & LOOKUP_RCU)
+		return -ECHILD;
+
 	if (direntry->d_inode) {
 		if (cifs_revalidate_dentry(direntry))
 			return 0;