nfsd4: use common rpc_cred for all callbacks

Callbacks are always made using the machine's identity, so we can use a
single auth_generic credential shared among callbacks to all clients and
let the rpc code take care of the rest.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 11db40c..0445192 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -696,10 +696,6 @@
 		clp->cl_cb_conn.cb_client = NULL;
 		rpc_shutdown_client(clnt);
 	}
-	if (clp->cl_cb_conn.cb_cred) {
-		put_rpccred(clp->cl_cb_conn.cb_cred);
-		clp->cl_cb_conn.cb_cred = NULL;
-	}
 }
 
 static inline void
@@ -4020,7 +4016,7 @@
 		return -ENOMEM;
 	queue_delayed_work(laundry_wq, &laundromat_work, grace_time);
 	set_max_delegations();
-	return 0;
+	return set_callback_cred();
 }
 
 int