Finish bf_update_range_ok
Finish bf_update_range_ok

Added length preservation from bf_put_chunk_ok to bf_update_range loop
invariant (hidden, since it confuses substitutions), which handles the
last admit in bf_update_range_ok.

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