NFSv4: Fix open create exclusive when the server reboots

If the server that does not implement NFSv4.1 persistent session
semantics reboots while we are performing an exclusive create,
then the return value of NFS4ERR_DELAY when we replay the open
during the grace period causes us to lose the verifier.
When the grace period expires, and we present a new verifier,
the server will then correctly reply NFS4ERR_EXIST.

This commit ensures that we always present the same verifier when
replaying the OPEN.

Reported-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>