builtin/fsck: refactor to use `odb_for_each_object()`

builtin/fsck: refactor to use `odb_for_each_object()`

In git-fsck(1) we have two callsites where we iterate over all objects
via `for_each_loose_object()` and `for_each_packed_object()`. Both of
these are trivially convertible with `odb_for_each_object()`.

Refactor these callsites accordingly.

Note that `odb_for_each_object()` may iterate over the same object
multiple times, for example when it exists both in packed and loose
format. But this has already been the case beforehand, so this does not
result in a change in behaviour.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>