f2fs: spread struct f2fs_dentry_ptr for inline path

Use f2fs_dentry_ptr structure to indicate inline dentry structure as
much as possible, so we can wrap inline dentry with size-fixed fields
to the one with size-changeable fields. With this change, we can
handle size-changeable inline dentry more easily.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>