f2fs: give correct trimmed blocks in fstrim

We have supported to issue discard in specified range during fstrim,
it needs to return caller with successfully trimmed bytes in that
range instead of bytes of invalid blocks which are scanned in
checkpoint.

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