NFSD: Reset cb_seq_status after NFS4ERR_DELAY

I noticed that once an NFSv4.1 callback operation gets a
NFS4ERR_DELAY status on CB_SEQUENCE and then the connection is lost,
the callback client loops, resending it indefinitely.

The switch arm in nfsd4_cb_sequence_done() that handles
NFS4ERR_DELAY uses rpc_restart_call() to rearm the RPC state machine
for the retransmit, but that path does not call the rpc_prepare_call
callback again. Thus cb_seq_status is set to -10008 by the first
NFS4ERR_DELAY result, but is never set back to 1 for the retransmits.

nfsd4_cb_sequence_done() thinks it's getting nothing but a
long series of CB_SEQUENCE NFS4ERR_DELAY replies.

Fixes: 7ba6cad6 ("nfsd: New helper nfsd4_cb_sequence_done() for processing more cb errors")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4callback.c

@@ -1178,6 +1178,7 @@ static bool nfsd4_cb_sequence_done(struct rpc_task *task, struct nfsd4_callback
 		ret = false;
 		break;
 	case -NFS4ERR_DELAY:
+		cb->cb_seq_status = 1;
 		if (!rpc_restart_call(task))
 			goto out;