fsck-objects: support platforms without d_ino in struct dirent.

fsck-objects: support platforms without d_ino in struct dirent.

The d_ino field is only used for performance reasons in
fsck-objects.  On a typical filesystem, i-number tends to have a
strong correlation with where the actual bits sit on the disk
platter, and we sort the entries to allow us scan things that
ought to be close together together.

If the platform lacks support for it, it is not a big deal.
Just do not use d_ino for sorting, and scan them unsorted.

Signed-off-by: Junio C Hamano <junkio@cox.net>