Revert "tweak the WritebackCache rep so that the evict_ok spec is simpler"
Revert "tweak the WritebackCache rep so that the evict_ok spec is simpler"

Maybe not a good idea, because cachepred's vs is no longer the superset
of what's on the real disk.

This reverts commit c57fc35341b5811cc7b7a71f36f9fc82d7a2e587.

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl