For now, the IVs are constant. This is fixed in a later commit.
They are enabled by default to make testing easier.
The whole thing passes the test suite on x86 and x86_64.
Disable block cache (in EncFS) and stat cache (in kernel).
This is needed if the backing files may be modified
behind the back of EncFS (for example, when you mount
an encrypted filesystem exported by encfs --reverse).
The reverse grow tests fail when this option is not passed to the
decrypting mount.
