f2fs: clean up the add_orphan_inode func

For the code
> prev = list_entry(orphan->list.prev, typeof(*prev), list);
if orphan->list.prev == head, it can't get the right prev.
And we can use the parameter 'this' to add.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>