fat: optimize fat_count_free_clusters()

EXPORT_SYMBOL_GPL(fat_free_clusters);

/* 128kb is the whole sectors for FAT12 and FAT16 */

#define FAT_READA_SIZE		(128 * 1024)

static void fat_ent_reada(struct super_block *sb, struct fat_entry *fatent,

			  unsigned long reada_blocks)

{

	struct fatent_operations *ops = MSDOS_SB(sb)->fatent_ops;

	sector_t blocknr;

	int i, offset;

	ops->ent_blocknr(sb, fatent->entry, &offset, &blocknr);

	for (i = 0; i < reada_blocks; i++)

		sb_breadahead(sb, blocknr + i);

}

int fat_count_free_clusters(struct super_block *sb)

{

	struct msdos_sb_info *sbi = MSDOS_SB(sb);

	struct fatent_operations *ops = sbi->fatent_ops;

	struct fat_entry fatent;

	unsigned long reada_blocks, reada_mask, cur_block;

	int err = 0, free;

	lock_fat(sbi);

	if (sbi->free_clusters != -1)

		goto out;

	reada_blocks = FAT_READA_SIZE >> sb->s_blocksize_bits;

	reada_mask = reada_blocks - 1;

	cur_block = 0;

	free = 0;

	fatent_init(&fatent);

	fatent_set_entry(&fatent, FAT_START_ENT);

	while (fatent.entry < sbi->max_cluster) {

		/* readahead of fat blocks */

		if ((cur_block & reada_mask) == 0) {

			unsigned long rest = sbi->fat_length - cur_block;

			fat_ent_reada(sb, &fatent, min(reada_blocks, rest));

		}

		cur_block++;

		err = fat_ent_read_block(sb, &fatent);

		if (err)

			goto out;