[$] Defending mounted filesystems from the root user

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