NFSv4.x: Enforce the ca_maxresponsesize_cached on the back channel

We have no duplicate reply cache, so we always set the back channel
ca_maxresponsesize_cached to zero when negotiating the session.
That means we should always error out as soon as we see the server
set args->csa_cachethis.
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/callback_proc.c

@@ -381,9 +381,8 @@ validate_seqid(struct nfs4_slot_table *tbl, struct cb_sequenceargs * args)
 		if (args->csa_cachethis == 0)
 			return htonl(NFS4ERR_RETRY_UNCACHED_REP);
 
-		/* The ca_maxresponsesize_cached is 0 with no DRC */
-		else if (args->csa_cachethis == 1)
-			return htonl(NFS4ERR_REP_TOO_BIG_TO_CACHE);
+		/* Liar! We never allowed you to set csa_cachethis != 0 */
+		return htonl(NFS4ERR_SEQ_FALSE_RETRY);
 	}
 
 	/* Wraparound */
@@ -500,6 +499,10 @@ __be32 nfs4_callback_sequence(struct cb_sequenceargs *args,
 
 	cps->slotid = args->csa_slotid;
 
+	/* The ca_maxresponsesize_cached is 0 with no DRC */
+	if (args->csa_cachethis != 0)
+		return htonl(NFS4ERR_REP_TOO_BIG_TO_CACHE);
+
 	/*
 	 * Check for pending referring calls.  If a match is found, a
 	 * related callback was received before the response to the original