Fix impl and spec for bf_put_chunk
Fix impl and spec for bf_put_chunk

It was so wrong in so many ways, very much promising not to do anything
(which seemed fine earlier since update_chunk did indeed not do
anything).

"step. step. step." proof still goes through, though, which is awesome.

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