NFSv4: Fix a hang in OPEN related to server reboot

NFSv4: Fix a hang in OPEN related to server reboot

If the server fails to return the attributes as part of an OPEN
reply, and then reboots, we can end up hanging. The reason is that
the client attempts to send a GETATTR in order to pick up the
missing OPEN call, but fails to release the slot first, causing
reboot recovery to deadlock.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Fixes: 2e80dbe7ac51a ("NFSv4.1: Close callback races for OPEN, LAYOUTGET...")
Cc: stable@vger.kernel.org # v4.8+