f2fs: fix to detect looped node chain correctly

Below dmesg was printed when testing generic/388 of fstest:

F2FS-fs (zram1): find_fsync_dnodes: detect looped node chain, blkaddr:526615, next:526616
F2FS-fs (zram1): Cannot recover all fsync data errno=-22
F2FS-fs (zram1): Mounted with checkpoint version = 22300d0e
F2FS-fs (zram1): find_fsync_dnodes: detect looped node chain, blkaddr:526615, next:526616
F2FS-fs (zram1): Cannot recover all fsync data errno=-22

The reason is that we initialize free_blocks with free blocks of
filesystem, so if filesystem is full, free_blocks can be zero,
below condition will be true, so that, it will fail recovery.

if (++loop_cnt >= free_blocks ||
	blkaddr == next_blkaddr_of_node(page))

To fix this issue, initialize free_blocks with correct value which
includes over-privision blocks.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>