f2fs: add key function to handle inline dir

Adds Functions to implement inline dir init/lookup/insert/delete/convert ops.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
[Jaegeuk Kim: remove needless reserved area copy, pointed by Dan Carpenter]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>