[$] Preventing information leaks from ext4 filesystems

A filesystem's role is to store information and retrieve it in its originalform on request. But filesystems are also expected to prevent theretrieval of information by people who should not see it. That requirementextends to data that has been deleted; users expect that data to be trulygone and will not welcome its reappearance in surprising places. Some workbeing done with ext4 shows the kind of measures that are required to liveup to that expectation.