make simple_lookup() usable for filesystems that set ->s_d_op

Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/libfs.c

@@ -61,7 +61,8 @@ struct dentry *simple_lookup(struct inode *dir, struct dentry *dentry, unsigned
 
 	if (dentry->d_name.len > NAME_MAX)
 		return ERR_PTR(-ENAMETOOLONG);
-	d_set_d_op(dentry, &simple_dentry_operations);
+	if (!dentry->d_sb->s_d_op)
+		d_set_d_op(dentry, &simple_dentry_operations);
 	d_add(dentry, NULL);
 	return NULL;
 }