f2fs: clean up checkpoint flow

This patch cleans up checkpoint flow a bit:
- remove unneeded circulation of flushing meta pages.
- don't flush nat_bits pages in prior to other checkpoint pages.
- add bug_on to check remained meta pages after flushing.

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