bcachefs: Repair code for directory i_size

We had a bug due due to an incomplete revert of the patch implementing
directory i_size (summing up the size of the dirents), leading to
completely screwy i_size values that underflow.

Most userspace programs don't seem to care (e.g. du ignores it), but it
turns out this broke sshfs, so needs to be repaired.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>