f2fs: fix to migrate blocks correctly during defragment

During defragment, we missed to trigger fragmented blocks migration
for below condition:

In defragment region:
- total number of valid blocks is smaller than 512;
- the tail part of the region are all holes;

In addtion, return zero to user via range->len if there is no
fragmented blocks.

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