FolderMemoryCard: Make sure data within a block is written in-order.

Helpful so we don't get silly things like someone erasing the cluster with the indirect FAT and then writing FAT values before restoring the indirect FAT, that kinda stuff.
FolderMemoryCard: Writes should also be possible to file entries.
FolderMemoryCard: Keep writes to locations we can't actually map to anything in memory and apply them once we do have that information.
FolderMemoryCard: Work around half-written file entries.

Apparently it is possible for applications to reserve a file entry but
not fully populate it with accurate data yet.