f2fs: avoid needless checkpoint in f2fs_trim_fs

The f2fs_trim_fs() doesn't need to do checkpoint if there are newly allocated
data blocks only which didn't change the critical checkpoint data such as nat
and sit entries.

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