bcachefs: make directory i_size meaningful

bcachefs: make directory i_size meaningful

The isize of directory is 0 in bcachefs if the directory is empty.
With more child dirents created, its size ought to change. Many
other filesystems changed as that (ie. xfs and btrfs). And many of
them changed as the size of child dirent name. Although the directory
size may not seem to convey much, we can still give it some meaning.

The formula of dentry size as follow:
    occupied_size = 40 + ALIGN(9 + namelen, 8)

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>