ovl: support stacked SEEK_HOLE/SEEK_DATA

static loff_t ovl_llseek(struct file *file, loff_t offset, int whence)

{

	struct inode *realinode = ovl_inode_real(file_inode(file));

	struct inode *inode = file_inode(file);

	struct fd real;

	const struct cred *old_cred;

	ssize_t ret;

	/*

	 * The two special cases below do not need to involve real fs,

	 * so we can optimizing concurrent callers.

	 */

	if (offset == 0) {

		if (whence == SEEK_CUR)

			return file->f_pos;

		if (whence == SEEK_SET)

			return vfs_setpos(file, 0, 0);

	}

	ret = ovl_real_fdget(file, &real);

	if (ret)

		return ret;

	return generic_file_llseek_size(file, offset, whence,

					realinode->i_sb->s_maxbytes,

					i_size_read(realinode));

	/*

	 * Overlay file f_pos is the master copy that is preserved

	 * through copy up and modified on read/write, but only real

	 * fs knows how to SEEK_HOLE/SEEK_DATA and real fs may impose

	 * limitations that are more strict than ->s_maxbytes for specific

	 * files, so we use the real file to perform seeks.

	 */

	inode_lock(inode);

	real.file->f_pos = file->f_pos;

	old_cred = ovl_override_creds(inode->i_sb);

	ret = vfs_llseek(real.file, offset, whence);

	revert_creds(old_cred);

	file->f_pos = real.file->f_pos;

	inode_unlock(inode);

	fdput(real);

	return ret;

}

static void ovl_file_accessed(struct file *file)