Fix bug in cache write
Fix bug in cache write

Writes would not update the cache contents if the block was already
in the cache.  That's what I get for putting off proofs to later...

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