NFSD: CREATE_SESSION must never cache NFS4ERR_DELAY replies

NFSD: CREATE_SESSION must never cache NFS4ERR_DELAY replies

There are one or two cases where CREATE_SESSION returns
NFS4ERR_DELAY in order to force the client to wait a bit and try
CREATE_SESSION again. However, after commit e4469c6cc69b ("NFSD: Fix
the NFSv4.1 CREATE_SESSION operation"), NFSD caches that response in
the CREATE_SESSION slot. Thus, when the client resends the
CREATE_SESSION, the server always returns the cached NFS4ERR_DELAY
response rather than actually executing the request and properly
recording its outcome. This blocks the client from making further
progress.

RFC 8881 Section 15.1.1.3 says:
> If NFS4ERR_DELAY is returned on an operation other than SEQUENCE
> that validly appears as the first operation of a request ... [t]he
> request can be retried in full without modification. In this case
> as well, the replier MUST avoid returning a response containing
> NFS4ERR_DELAY as the response to an initial operation of a request
> solely on the basis of its presence in the reply cache.

Neither the original NFSD code nor the discussion in section 18.36.4
refer explicitly to this important requirement, so I missed it.

Note also that not only must the server not cache NFS4ERR_DELAY, but
it has to not advance the CREATE_SESSION slot sequence number so
that it can properly recognize and accept the client's retry.

Reported-by: Dai Ngo <dai.ngo@oracle.com>
Fixes: e4469c6cc69b ("NFSD: Fix the NFSv4.1 CREATE_SESSION operation")
Tested-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>