f2fs: use extent cache for dir

We update extent cache for all user inode of f2fs including dir inode, so this
patch gives another chance to try to get physical address of page from extent
cache for dir inode.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>