[$] Defending mounted filesystems from the root user

Making a filesystem implementation robust in the face of maliciouslycreated filesystem images is a challenging task even when theimplementation is actively maintained, which many in the kernel are not. There is a way tomake that task even harder, though: modify that filesystem image behind theimplementation's back while it is mounted. A recent discussion on thelinux-fsdevel list reveals an ongoing disagreement over whether (and how)this threat should be addressed.