rcu: Don't bother affinitying rcub kthreads away from offline CPUs

When rcu_boost_kthread_setaffinity() sees that all CPUs for a given
rcu_node structure are now offline, it affinities the corresponding
RCU-boost ("rcub") kthread away from those CPUs.  This is pointless
because the kthread cannot run on those offline CPUs in any case.
This commit therefore removes this unneeded code.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree_plugin.h

@@ -1322,12 +1322,8 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 	for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask >>= 1)
 		if ((mask & 0x1) && cpu != outgoingcpu)
 			cpumask_set_cpu(cpu, cm);
-	if (cpumask_weight(cm) == 0) {
+	if (cpumask_weight(cm) == 0)
 		cpumask_setall(cm);
-		for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++)
-			cpumask_clear_cpu(cpu, cm);
-		WARN_ON_ONCE(cpumask_weight(cm) == 0);
-	}
 	set_cpus_allowed_ptr(t, cm);
 	free_cpumask_var(cm);
 }