NFSv4.1/pnfs: Play safe w.r.t. close() races when return-on-close is set

If we have an OPEN_DOWNGRADE and CLOSE race with one another, we want
to ensure that the layout is forgotten by the client, so that we
start afresh with a new layoutget.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>