bcachefs: Revert directory i_size

This turned out to have several bugs, which were missed because the fsck
code wasn't properly reporting errors - whoops.

Kicking it out for now, hopefully it can make 6.15.

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