bcachefs: Fix error type in bch2_alloc_v3_validate()

Use error type alloc_v3_unpack_error in bch2_alloc_v3_validate().

Fixes: b65db750 ("bcachefs: Enumerate fsck errors")
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>