f2fs: move dir data flush to write checkpoint process

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>

fs/f2fs/file.c

@@ -216,6 +216,9 @@ static int f2fs_do_sync_file(struct file *file, loff_t start, loff_t end,
 
 	trace_f2fs_sync_file_enter(inode);
 
+	if (S_ISDIR(inode->i_mode))
+		goto go_write;
+
 	/* if fdatasync is triggered, let's do in-place-update */
 	if (datasync || get_dirty_pages(inode) <= SM_I(sbi)->min_fsync_blocks)
 		set_inode_flag(inode, FI_NEED_IPU);