makefs: use unsigned and %u for rock_ridge_move_count

For diff reduction against NetBSD's makefs. Based on NetBSD git mirror
commit 00991aee8248.

With this change our makefs cd9660 support should be in sync up to
NetBSD commit bdae6c9dc792 ("makefs(8): Nix trailing whitespace.").

Sponsored by: The FreeBSD Foundation