Fix the file_truncate_recover_ok proof
Fix the file_truncate_recover_ok proof

Was hitting an evar-related bug due to instantiating an evar with a
value produced after it was created. Instantiation should have failed
much earlier at eassign_idempred, or Coq should have cleverly worked
around the issue, but this version is safer regardless.

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