fsck: use enum object_type for fsck_walk callback

fsck: use enum object_type for fsck_walk callback

We switched the function interface for fsck callbacks in a1aad71601
(fsck.h: use "enum object_type" instead of "int", 2021-03-28). However,
we accidentally flipped the type back to "int" as part of 0b4e9013f1
(fsck: mark unused parameters in various fsck callbacks, 2023-07-03).
The mistake happened because that commit was written before a1aad71601
and rebased forward, and I screwed up while resolving the conflict.

Curiously, the compiler does not warn about this mismatch, at least not
when using gcc and clang on Linux (nor in any of our CI environments).
Based on 28abf260a5 (builtin/fsck.c: don't conflate "int" and "enum" in
callback, 2021-06-01), I'd guess that this would cause the AIX xlc
compiler to complain. I noticed because clang-18's UBSan now identifies
mis-matched function calls at runtime, and does complain of this case
when running the test suite.

I'm not entirely clear on whether this mismatch is a problem in
practice. Compilers are certainly free to make enums smaller than "int"
if they don't need the bits, but I suspect that they have to promote
back to int for function calls (though I didn't dig in the standard, and
I won't be surprised if I'm simply wrong and the real-world impact would
depend on the ABI).

Regardless, switching it back to enum is obviously the right thing to do
here; the switch to "int" was simply a mistake.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>