bcachefs: Fix fsck directory i_size checking

Error handling was wrong, causing unhandled transaction restart errors.

check_directory_size() was also inefficient, since keys in multiple
snapshots would be iterated over once for every snapshot. Convert it to
the same scheme used for i_sectors and subdir count checking.

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