rcu: Substitute lookup for bit-twiddling in sync_rcu_exp_select_node_cpus()

The code in sync_rcu_exp_select_node_cpus() calculates the current
CPU's mask within its rcu_node structure's bitmasks, but this has
already been computed in the ->grpmask field of that CPU's rcu_data
structure.  This commit therefore just uses this ->grpmask field.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_exp.h

@@ -345,8 +345,8 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
 	/* Each pass checks a CPU for identity, offline, and idle. */
 	mask_ofl_test = 0;
 	for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
-		unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
 		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+		unsigned long mask = rdp->grpmask;
 		int snap;
 
 		if (raw_smp_processor_id() == cpu ||
@@ -373,8 +373,8 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
 
 	/* IPI the remaining CPUs for expedited quiescent state. */
 	for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi) {
-		unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
 		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+		unsigned long mask = rdp->grpmask;
 
 retry_ipi:
 		if (rcu_dynticks_in_eqs_since(rdp, rdp->exp_dynticks_snap)) {