ovl: add O_DIRECT support

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/file.c

@@ -110,6 +110,9 @@ static int ovl_open(struct inode *inode, struct file *file)
 	if (IS_ERR(realfile))
 		return PTR_ERR(realfile);
 
+	/* For O_DIRECT dentry_open() checks f_mapping->a_ops->direct_IO */
+	file->f_mapping = realfile->f_mapping;
+
 	file->private_data = realfile;
 
 	return 0;