ovl: improving copy-up efficiency for big sparse file

	loff_t old_pos = 0;

	loff_t new_pos = 0;

	loff_t cloned;

	loff_t data_pos = -1;

	loff_t hole_len;

	bool skip_hole = false;

	int error = 0;

	if (len == 0)

		goto out;

	/* Couldn't clone, so now we try to copy the data */

	/* FIXME: copy up sparse files efficiently */

	/* Check if lower fs supports seek operation */

	if (old_file->f_mode & FMODE_LSEEK &&

	    old_file->f_op->llseek)

		skip_hole = true;

	while (len) {

		size_t this_len = OVL_COPY_UP_CHUNK_SIZE;

		long bytes;

			break;

		}

		/*

		 * Fill zero for hole will cost unnecessary disk space

		 * and meanwhile slow down the copy-up speed, so we do

		 * an optimization for hole during copy-up, it relies

		 * on SEEK_DATA implementation in lower fs so if lower

		 * fs does not support it, copy-up will behave as before.

		 *

		 * Detail logic of hole detection as below:

		 * When we detect next data position is larger than current

		 * position we will skip that hole, otherwise we copy

		 * data in the size of OVL_COPY_UP_CHUNK_SIZE. Actually,

		 * it may not recognize all kind of holes and sometimes

		 * only skips partial of hole area. However, it will be

		 * enough for most of the use cases.

		 */

		if (skip_hole && data_pos < old_pos) {

			data_pos = vfs_llseek(old_file, old_pos, SEEK_DATA);

			if (data_pos > old_pos) {

				hole_len = data_pos - old_pos;

				len -= hole_len;

				old_pos = new_pos = data_pos;

				continue;

			} else if (data_pos == -ENXIO) {

				break;

			} else if (data_pos < 0) {

				skip_hole = false;

			}

		}

		bytes = do_splice_direct(old_file, &old_pos,

					 new_file, &new_pos,

					 this_len, SPLICE_F_MOVE);

	}

	inode_lock(temp->d_inode);

	if (c->metacopy)

	if (S_ISREG(c->stat.mode))

		err = ovl_set_size(temp, &c->stat);

	if (!err)

		err = ovl_set_attr(temp, &c->stat);