f2fs: move dir data flush to write checkpoint process

[ Upstream commit b61ac5b720146c619c7cdf17eff2551b934399e5 ]

This patch move dir data flush to write checkpoint process, by
doing this, it may reduce some time for dir fsync.

pre:
	-f2fs_do_sync_file enter
		-file_write_and_wait_range  <- flush & wait
		-write_checkpoint
			-do_checkpoint	    <- wait all
	-f2fs_do_sync_file exit

now:
	-f2fs_do_sync_file enter
		-write_checkpoint
			-block_operations   <- flush dir & no wait
			-do_checkpoint	    <- wait all
	-f2fs_do_sync_file exit

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>